Cell line comprising vector encoding truncated FLK-1 receptor

ABSTRACT

The present invention relates to the use of ligands for the FLK-1 receptor for the modulation of angiogenesis and vasculogenesis. The invention s based, in part, on the demonstration that Flk-1 tyrosine kinase receptor expression is associated with endothelial cells and the identification of vascular endothelial growth factor (VEGF) as the high affinity ligand of Flk-1. These results indicate a major role for Flk-1 in the signaling system during vasculogenesis and angiogenesis. Engineering of host cells that express Flk-1 and the uses of expressed Flk-1 to evaluate and screen for drugs and analogs of VEGF involved in Flk-1 modulation by either agonist or antagonist activities is described. The invention also relates to the use of FLK-1 ligands, including VEGF agonists and antagonists, in the treatment of disorders, including cancer, by modulating vasculogenesis and angiogenesis.

This application is a divisional of U.S. patent application Ser. No.09/766,678, filed Jan. 23, 2001, now U.S. Pat. No. 6,872,699, which is acontinuation of U.S. patent application Ser. No. 08/965,598 filed, Nov.6, 1997, now abandoned, which is a continuation of U.S. patentapplication Ser. No. 08/193,829, filed Feb. 9, 1994, now U.S. Pat. No.6,177,401, which is a continuation-in-part of U.S. patent applicationSer. No. 08/038,596, filed Mar. 26, 1993, now abandoned, which is acontinuation-in-part of U.S. patent application Ser. No. 07/975,750,filed Nov. 13, 1992, now abandoned, all of which are incorporated byreference herein in their entireties.

1. INTRODUCTION

The present invention relates to the use of proteins, peptides andorganic molecules capable of modulating FLK-1 receptor signaltransduction in order to inhibit or promote angiogenesis andvasculogenesis. The invention is based, in part, on the demonstrationthat Flk-1 tyrosine kinase receptor expression is associated withendothelial cells and the identification of vascular endothelial growthfactor (VEGF) as a high affinity ligand of Flk-1. These results indicatea major role for Flk-1 in the signaling system during vasculogenesis andangiogenesis. Engineering of host cells that express Flk-1 and the usesof expressed Flk-1 to evaluate and screen for drugs and analogs of VEGFinvolved in Flk-1 modulation by either agonist or antagonist activitiesis described.

The invention also relates to the use of FLK-1 ligands, including VEGFagonists and antagonists, in the treatment of disorders, includingcancer, by modulating vasculogenesis and angiogenesis.

2. BACKGROUND OF THE INVENTION

Receptor tyrosine kinases comprise a large family of transmembranereceptors for polypeptide growth factors with diverse biologicalactivities. Their intrinsic tyrosine kinase function is activated uponligand binding, which results in phosphorylation of the receptor andmultiple cellular substrates, and subsequently in a variety of cellularresponses (Ullrich A. and Schlessinger, J., 1990, Cell 61:203-212).

A receptor tyrosine kinase cDNA, designated fetal liver kinase 1(Flk-1), was cloned from mouse cell populations enriched forhematopoietic stem and progenitor cells. The receptor was suggested tobe involved in hematopoietic stem cell renewal (Matthews et al., 1991,Proc. Natl. Acad. Sci. USA 88:9026-9030). Sequence analysis of the Flk-1clone revealed considerable homology with the c-Kit subfamily ofreceptor kinases and in particular to the Flt gene product. Thesereceptors all have in common an extracellular domain containingimmunoglobulin-like structures.

The formation and spreading of blood vessels, or vasculogenesis andangiogenesis, respectively, play important roles in a variety ofphysiological processes such as embryonic development, wound healing,organ regeneration and female reproductive processes such as follicledevelopment in the corpus luteum during ovulation and placental growthafter pregnancy. Uncontrolled angiogenesis can be pathological such asin the growth of solid tumors that rely on vascularization for growth.

Angiogenesis involves the proliferation, migration and infiltration ofvascular endothelial cells, and is likely to be regulated by polypeptidegrowth factors. Several polypeptides with in vitro endothelial cellgrowth promoting activity have been identified. Examples include acidicand basic fibroblastic growth factor, vascular endothelial growth factorand placental growth factor. Although four distinct receptors for thedifferent members of the FGF family have been characterized, none ofthese have as yet been reported to be expressed in blood vessels invivo.

While the FGFs appear to be mitogens for a large number of differentcell types, VEGF has recently been reported to be an endothelial cellspecific mitogen (Ferrara, N. and Henzel, W. J., 1989, Biochem. Biophys.Res. Comm. 161:851-858). Recently, the fms-like tyrosine receptor, fit,was shown to have affinity for VEGF (DeVries, C. et al., 1992, Science255:989-991).

3. SUMMARY OF THE INVENTION

The present invention relates to the use of peptides, proteins andorganic molecules capable of modulating FLK-1 receptor signaltransduction in order to inhibit or promote angiogenesis and/orvasculogenesis. The present invention is based, in part, on thediscovery that the Flk-1 tyrosine kinase receptor is expressed on thesurface of endothelial cells and the identification of vascularendothelial growth factor (VEGF) as a high affinity ligand of Flk-1. Therole of endothelial cell proliferation and migration during angiogenesisand vasculogenesis indicate an important role for Flk-1 in theseprocesses. The invention is described by way of example for the murineFlk-1, however, the principles may be applied to other species includinghumans.

Pharmaceutical reagents designed to inhibit the Flk-1/VEGF interactionmay be useful in inhibition of tumor growth. VEGF and/or VEGF agonistsmay be used to promote wound healing. The invention relates toexpression systems designed to produce Flk-1 protein and/or cell lineswhich express the Flk-1 receptor. Expression of soluble recombinantFlk-1 protein may be used to screen peptide libraries for molecules thatinhibit the Flk-1/VEGF interaction. Engineered cell lines expressingFlk-1 on their surface may be advantageously used to screen and identifyVEGF agonists and antagonists.

4. BRIEF DESCRIPTION OF THE FIGURES

FIG. 1. Comparison of the Flk-1 amino acid sequence with related RTKs.Amino acid sequence comparison of Flk-1 (SEQ ID NO: 9) with human KDR(SEQ ID NO: 10) and rat TKr-C (SEQ ID NO: 11). A section of the sequencewhich is known for all three receptors is compared and only differencesto the Flk-1 sequence are shown.

FIGS. 2A and 2B. Northern blot analysis of Flk-1 gene expression. FIG.2A. Expression of Flk-1 RNA in day 9.5 to day 18.5 mouse embryos.Samples (10 μg) of total RNA from whole mouse embryos were analyzed ineach lane. Positions of 28S and 18S ribosomal RNAs are marked.

FIG. 2B Expression of Flk-1 mRNA in postnatal day 4 and adult brain incomparison with capillary fragments from postnatal day 4 brain. 1 μg ofpoly (A⁺) RNA was loaded on each lane. The 5′ 2619 bp of the Flk-1 cDNAwere used as a probe. Control hybridization with a GAPDH cDNA probe isshown in the lower panel.

FIGS. 3A, 3B and 3C. Abundant Flk-1 gene expression in embryonictissues. In situ hybridization analysis of Flk-1 expression in day 14.5mouse embryo. FIG. 3A Bright field illumination of a parasagittalsection through the whole embryo hybridized with a ³⁵S-labeled antisenseprobe (5′ 2619 bp). FIG. 3B Dark field illumination of the same section.FIG. 3C Control hybridization of an adjacent section with a sense probe.Abbreviations: Ao, aorta; At, atrium; L, lung; Li, liver; Ma, mandible;Mn, meninges; Ms. mesencephalon; T, telencephalon; V, ventricle; Vt,vertebrae.

FIGS. 4A, 4B, 4C, 4D and 4E. Expression of Flk-1 RNA in embryonic organsis restricted to specific cells. Expression of Flk-1 RNA in a day 14.5mouse embryo at higher magnification. FIG. 4A The heart region wasprobed with a ³⁵S-labeled antisense probe. FIG. 4B Adjacent sectionhybridized with the sense probe.

FIG. 4C Part of the aorta wall shown on the cellular level. Theendothelial cell-layer is indicated by an arrow. FIG. 4D The lung,probed with the Flk-1 antisense probe. FIG. 4E Control hybridization ofan adjacent section hybridized with the sense probe. Abbreviations: At,atrium; B, bronchus; Ed, endothelial cell layer; En, endocardium; L,lung, Li, liver; Lu, lumina of the aorta; Ml, muscular; My, myocardium.

FIGS. 5A, 5B, 5C and 5D. Flk-1 gene expression in the brain of thedeveloping mouse. In situ hybridization analysis of Flk-1 geneexpression in the brain at different developmental stages. All sectionswere probed with the Flk-1 antisense probe. FIG. 5A Sagittal section ofthe telencephalon of a day 11.5 mouse embryo. A single blood vesselexpressing Flk-1, which sprouts from the meninges into theneuroectoderm, is indicated by an arrow. FIG. 5B Sagittal sections ofthe brain of embryo day 14.5 and FIG. 5C of postnatal day 4. Shown areregions of the mesencephalon. Branching capillaries and blood vesselsexpressing Flk-1 are indicated by an arrow. FIG. D Sagittal section ofan adult brain; a region of the mesencephalon is shown. Cells expressingFlk-1 are indicated by an arrow. Abbreviations: M, meninges; V,ventricle;

FIGS. 6A and 6B. Expression of Flk-1 in the choroid plexus of adultbrain. FIG. 6A Darkfield illumination of the choroid plexus of an adultmouse brain hybridized with Flk-1 antisense probe. FIG. 6B Choroidplexus shown at a higher magnification. Arrows indicate single cells,which show strong expression of Flk-1. Abbreviations: CP, choroidplexus; E, ependyme; Ep, epithelial cells; V, ventricle.

FIGS. 7A, 7B, 7C and 7D. Flk-1 is expressed in the glomeruli of thekidney. FIG. 7A Parasagittal section of a 4-day postnatal kidney,hybridized with the Flk-1 antisense probe. Hybridization signalaccumulates in the glomeruli, as indicated by arrowheads. FIG. 7BControl hybridization of an adjacent section with the sense probe. FIG.7C Sagittal section of an adult kidney probed with Flk-1. Arrowheadsindicate glomeruli. FIG. 7D Glomerulus of an adult kidney at a highermagnification. The arrows in (A) and (D) indicate cells aligned instrands in the juxtaglomerular region expressing Flk-1.

FIGS. 8A, 8 b, 8C and 8D. In situ hybridization analysis of Flk-1expression in early embryos and extraembryonic tissues. FIG. 8A Sagittalsection of a day 8.5 mouse embryo in the maternal deciduum probed withFlk-1. FIG. 8B Higher magnification of the deciduum. Arrowheads indicatethe endothelium of maternal blood vessels strongly expressing Flk-1 RNA.FIG. 8C High magnification of the yolk sac and the trophectoderm of aday 9.5 mouse embryo. FIG. 8D High magnification of a blood island.Abbreviations: A, allantois; Bi, blood island; By, maternal bloodvessel; D, deciduum; En, endodermal layer of yolk sac; M, mesenchyme;Ms, mesodermal layer of yolk sac; NF, neural fold; T, trophoblast; Y.yolk sac.

FIGS. 9A and 9B. Flk-1 is a receptor for VEGF. FIG. 9A Cross linking of¹²⁵I-VEGF to COS cells transiently expressing the Flk-1 receptor andcontrol cells were incubated with ¹²⁵I-VEGF at 4° C. overnight, thenwashed twice with phosphate buffered saline (PBS) and exposed to 0.5 mMof the cross linking agent DSS in PBS for 1 hour at 4° C. The cells werelysed, Flk-1 receptor immunoprecipitated, and analyzed by polyacrylamidegel electrophoresis followed by autoradiography. Molecular size markersare indicated in kilodaltons. FIG. 9B Specific binding of ¹²⁵I-VEGF toCOS cells expressing Flk-1. COS cells transiently expressing Flk-1 wereremoved from the plate and resuspended in binding medium (DMEM, 25 mMHepes, 0.15% gelatin). Binding was performed at 15° C. for 90 minutes ina total volume of 0.5 ml containing 2×10⁵ cells, 15,000 cpm ¹²⁵I-VEGF,and the indicated concentrations of unlabeled ligand. The cells werewashed twice with PBS/0.1% BSA and counted in a gamma counter.

FIG. 10. VEGF-induced autophosphorylation of Flk-1. COS cellstransiently expressing Flk-1 receptor and control cells were starved for24 hours in DMEM containing 0.5% fetal calf serum and then stimulatedwith VEGF for 10 minutes as indicated. The cells were solubilized, Flk-1receptor immunoprecipitated with a polyclonal antibody against itsC-terminus, separated by polyacrylamide gel electrophoresis, andtransferred to nitrocellulose. The blot was probed withantiphosphotyrosine antibodies (5B2). The protein bands were visualizedby using a horseradish-peroxidase coupled secondary antibody and BCL™(Amersham) detection assay.

FIGS. 11-1, 11-2, 11-3 and 11-4. Nucleotide sequence (SEQ ID NO: 7) andamino acid sequence (SEQ ID NO: 8) of murine Flk-1.

FIGS. 12A and 12B. Plasmid Maps of retroviral vector constructs. FIG.12A. pLXSN Flk-1 TM cl.1 and pLXSN Flk-1 TM cl.3, clonal isolates ofpLXSN Flk-1 TM, contain Flk-1 amino acids 1 through 806 and lack 561C-terminal amino acids of the intracellular kinase domain. FIG. 12B.pNTK-cfms-TM contains the 541 N-terminal amino acids of the CSF-1receptor/c-fms.

FIG. 13. Inhibition of C6 glioblastoma tumor growth bytransdominant-negative inhibition of Flk-1. C6 cells were implantedeither alone or coimplanted with virus-producing cells. Cell numbers areas indicated in each panel. Two different virus-producing cells lineswere used: one expressing the Flk-1 TM (transdominant-negative) mutantand one expressing a transdominant-negative c-fms mutant (c-fms TM) as acontrol. Beginning at the time when the first tumors appeared, tumorvolumes were measured every 2 to 3 days to obtain a growth curve. Eachgroup is represented by four mice.

FIG. 14. A second experiment showing inhibition of C6 glioblastoma tumorgrowth by transdominant-negative inhibition of Flk-1. C6 cells wereimplanted either alone or coimplanted with virus-producing cells. Cellnumbers are as indicated in each panel. Two different virus-producingcell lines were used: one expressing the Flk-1 TM(transdominant-negative) mutant and one expressing atransdominant-negative c-fms mutant (cfms TM) as a control. Beginning atthe time when the first tumor appeared, tumor volumes were measuredevery 2 to 3 days to obtain growth curve. Each group is represented byfour mice.

FIG. 15. Inhibition of C6 glioblastoma tumor growth by localizedinjection of retroviral supernatants. 1×10⁶ cells were subcutaneouslyimplanted in nude mice. Starting at day 5 after implantation (denoted bythe arrow), the growing tumors were treated by injection of 100 μlretroviral supernatants (about 105 virus particles) into the site oftumor implantation. Tumor volumes were measured twice a week.

FIGS. 16A and 16B. Inhibition of C6 glioblastoma tumor growth bylocalized injection of retroviral supernatants. C6 cells implantedintercranially in rats, either alone or co-implanted with virusproducing cells expressing the Flk-1 TM (transdominant negative) mutant.Each group is represented by 8 rats. Cell numbers are as indicated inExample 6.1.13.

FIG. 16A shows the distribution of tumor size in each rat. FIG. 16Bshows the median tumor area for each of the two groups of rats.

FIG. 17. Inhibition of VEGF stipulatory activity of Flk-1. Testcompounds and VEGF were co-incubated on cells expressing the Flk-1receptor. The level of tyrosine phosphorylation was measured in aWestern blot format using an antiphosphotyrosine antibody. Compound A14completely inhibited the ability of VEGF to stimulateautophosphorylation of Flk-1. (Compounds A7, A8, and A10 were toxic tothese cells resulting in cell death.).

5. DETAILED DESCRIPTION OF THE INVENTION

The present invention relates to the use of peptides, proteins, andorganic molecules capable of modulating FLK-1 receptor signaltransduction in order to inhibit or promote angiogenesis and/orvasculogenesis. More specifically, the invention is directed to VEGF, anatural ligand for Flk-1, as well as VEGF agonists and antagonists,anti-VEGF and anti-Flk-1 antibodies, and VEGF and Flk-1 encodingpolynucleotides, all of which may find use in modulating Flk-1 signaltransduction. Another aspect of the invention relates to the use ofFlk-1 expressing cells to evaluate and screen proteins, peptides, andorganic compounds that may be involved in Flk-1 receptor activation,regulation and/or uncoupling. Such regulators of Flk-1 may be usedtherapeutically. For example, agonists of VEGF may be used in processessuch as wound healing; in contrast, antagonists of VEGF may be used inthe treatment of tumors that rely on vascularization for growth.

The invention, is based, in part, on results from in situ-hybridizationand Northern blot analyses indicating that Flk-1 is an endothelial cellspecific RTK. In addition, cross-linking experiments have shown Flk-1 tobe a high affinity receptor for vascular endothelial growth factor(VEGF), indicating that Flk-1 plays a crucial role in the developmentand differentiation of hemangioblast and in subsequent endothelial cellgrowth during vasculogenesis and angiogenesis.

The invention is based, also, on the discovery that expression of atransdominant-negative mutant form of the Flk-1 molecule can inhibit thebiological activity of the endogenous wild type Flk-1. Experiments aredescribed herein, in which tumor cells and cells expressingretrovirally-encoded truncated, signalling-incompetent Flk-1 receptorwere injected into mice. In these experiments, the cells producing arecombinant retrovirus encoding a truncated Flk-1 receptor were eitherco-injected with the tumor cells or injected 5 days after injection ofthe tumor cells. Inhibition of vasculogenesis and growth of the injectedtumor cells was observed in mice expressing the truncated form of theFlk-1 receptor. Inhibition of tumor growth observed when the truncatedFlk-1 receptor was injected 5 days after injection of the tumor cellsindicates that even established tumors may be suppressed by Flk-1dominant-negative action. Thus, the invention provides a method ofinhibiting the biological activity of signalling-competent Flk-1receptors comprising introducing a signalling-incompetent Flk-1 receptormutant into or in the vicinity of cells expressing suchsignalling-competent Flk-1 receptors. Accordingly, expression oftransdominant negative forms of the Flk-1 molecule may be useful fortreatment of diseases resulting from VEGF and/or Flk-1 mediated,abnormal proliferation of blood vessels, such as rheumatoid arthritis,retinopathies and growth of solid tumors.

As explained in the working examples, infra, the polymerase chainreaction (PCR) method was used to isolate new receptor tyrosine kinasesspecifically expressed in post-implantation embryos and endothelialcells. One such clone was found to encode a RTK that had almostidentical sequence homology with the previously identified cDNA cloneisolated from populations of cells enriched for hematopoietic cells anddesignated fetal liver kinase-1 (Flk-1) (Matthews et al., 1991, Proc.Natl. Acad. Sci. U.S.A. 88:9026-9030) (FIGS. 11-1 through 11-4).

For clarity of discussion, the invention is described in the subsectionsbelow by way of example for the murine Flk-1. However, the principlesmay be analogously applied to clone and express the Flk-1 of otherspecies including humans.

5.1. THE Flk-1 Coding Sequence

The nucleotide coding sequence and deduced amino acid sequence of themurine Flk-1 gene is depicted in FIGS. 11-1 through 11-4 (SEQ ID NOs: 7and 8) and has recently been described in Matthews et al., 1991, Proc.Natl. Acad. Sci. U.S.A., 88: 9026-9030. In accordance with theinvention, the nucleotide sequence of the Flk-1 protein or itsfunctional equivalent in mammals, including humans, can be used togenerate recombinant molecules which direct the expression of Flk-1;hereinafter, this receptor will be referred to as “Flk-1”, regardless ofthe species from which it is derived.

In a specific embodiment described herein, the murine Flk-1 gene wasisolated by performing a polymerase chain reaction (PCR) using twodegenerate oligonucleotide primer pools that were designed on the basisof highly conserved sequences within the kinase domain of receptortyrosine kinases (Hanks et al., 1988). As a template, DNA from a λgt10cDNA library prepared from day 8.5 mouse embryos, was used. In aparallel approach, similar primers were used to amplify RTK cDNAsequences from capillary endothelial cells that had been isolated fromthe brains of post-natal day 4-8 mice. This is a time when brainendothelial cell proliferation is maximal. Both approaches yielded cDNAsequences encoding the recently described fetal liver RTK, Flk-1(Matthews et al., 1991). Based on amino acid homology, this receptor isa member of the type III subclass of RTKs (Ullrich and Schlessinger)which contain immunoglobulin-like repeats in their extracellular domains(FIG. 1).

The invention also relates to Flk-1 genes isolated from other species,including humans, in which Flk-1 activity exists. Members of the Flk-1family are defined herein as those receptors that bind VEGF or fragmentsof the peptide. Such receptors may demonstrate about 80% homology at theamino acid level in substantial stretches of DNA sequence. Abacteriophage cDNA library may be screened, under conditions of reducedstringency, using a radioactively labeled fragment of the mouse Flk-1clone. Alternatively the mouse Flk-1 sequence can be used to designdegenerate or fully degenerate oligonucleotide probes which can be usedas PCR probes or to screen bacteriophage cDNA libraries. A polymerasechain reaction (PCR) based strategy may be used to clone human Flk-1.Two pools of degenerate oligonucleotides, corresponding to a conservedmotif between the mouse Flk-1 and receptor tyrosine kinases, may bedesigned to serve as primers in a PCR reaction. The template for thereaction is cDNA obtained by reverse transcription of mRNA prepared fromcell lines or tissue known to express human Flk-1. The PCR product maybe subcloned and sequenced to insure that the amplified sequencesrepresent the Flk-1 sequences. The PCR fragment may be used to isolate afull length Flk-1 cDNA clone by radioactively labeling the amplifiedfragment and screening a bacteriophage cDNA library. Alternatively, thelabeled fragment may be used to screen a genomic library. For a reviewof cloning strategies which may be used, see e.g., Maniatis, 1989,Molecular Cloning, A Laboratory Manual, Cold Springs Harbor Press, N.Y.;and Ausubel et al., 1989, Current Protocols in Molecular Biology, (GreenPublishing Associates and Wiley Interscience, N.Y.)

Isolation of a human Flk-1 cDNA may also be achieved by construction ofa cDNA library in a mammalian expression vector such as pcDNA1, thatcontains SV40 origin of replication sequences which permit high copynumber expression of plasmids when transferred into COS cells. Theexpression of Flk-1 on the surface of transfected COS cells may bedetected in a number of ways, including the use of a labeled ligand suchas VEGF or a VEGF agonist labeled with a radiolabel, fluorescent labelor an enzyme. Cells expressing the human Flk-1 may be enriched bysubjecting transfected cells to a FACS (fluorescent activated cellsorter) sort.

In accordance with the invention, Flk-1 nucleotide sequences whichencode Flk-1, peptide fragments of Flk-1, Flk-1 fusion proteins orfunctional equivalents thereof may be used to generate recombinant DNAmolecules that direct the expression of Flk-1 protein or a functionalequivalent thereof, in appropriate host cells. Alternatively, nucleotidesequences which hybridize to portions of the Flk-1 sequence may also beused in nucleic acid hybridization assays, Southern and Northern blotanalyses, etc.

Due to the inherent degeneracy of the genetic code, other DNA sequenceswhich encode substantially the same or a functionally equivalent aminoacid sequence, may be used in the practice of the invention for thecloning and expression of the Flk-1 protein. Such DNA sequences includethose which are capable of hybridizing to the murine Flk-1 sequenceunder stringent conditions.

Altered DNA sequences which may be used in accordance with the inventioninclude deletions, additions or substitutions of different nucleotideresidues resulting in a sequence that encodes the same or a functionallyequivalent gene product. The gene product itself may contain deletions,additions or substitutions of amino acid residues within the Flk-1sequence, which result in a silent change thus producing a functionallyequivalent Flk-1. Such amino acid substitutions may be made on the basisof similarity in polarity, charge, solubility, hydrophobicity,hydrophilicity, and/or the amphipatic nature of the residues involved.For example, negatively charged amino acids include aspartic acid andglutamic acid; positively charged amino acids include lysine andarginine; amino acids with uncharged polar head groups having similarhydrophilicity values include the following: leucine, isoleucine,valine; glycine, analine; asparagine, glutamine; serine, threonine;phenylalanine, tyrosine. As used herein, a functionally equivalent Flk-1refers to a receptor which binds to VEGF or fragments, but notnecessarily with the same binding affinity of its counterpart nativeFlk-1.

The DNA sequences of the invention may be engineered in order to alterthe Flk-1 coding sequence for a variety of ends including but notlimited to alterations which modify processing and expression of thegene product. For example, mutations may be introduced using techniqueswhich are well known in the art, e.g. site-directed mutagenesis, toinsert new restriction sites, to alter glycosylation patterns,phosphorylation, etc. For example, in certain expression systems such asyeast, host cells may over glycosylate the gene product. When using suchexpression systems it may be preferable to alter the Flk-1 codingsequence to eliminate any N-linked glycosylation site.

In another embodiment of the invention, the Flk-1 or a modified Flk-1sequence may be ligated to a heterologous sequence to encode a fusionprotein. For example, for screening of peptide libraries it may beuseful to encode a chimeric Flk-1 protein expressing a heterologousepitope that is recognized by a commercially available antibody. Afusion protein may also be engineered to contain a cleavage site locatedbetween the Flk-1 sequence and the heterologous protein sequence, sothat the Flk-1 can be cleaved away from the heterologous moiety.

In an alternate embodiment of the invention, the coding sequence ofFlk-1 could be synthesized in whole or in part, using chemical methodswell known in the art. See, for example, Caruthers, et al., 1980, Nuc.Acids Res. Symp. Ser. 7:215-233; Crea and Horn, 180, Nuc. Acids Res.9(10):2331; Matteucci and Caruthers, 1980, Tetrahedron Letters 21:719;and Chow and Kempe, 1981, Nuc. Acids Res. 9(12):2807-2817.Alternatively, the protein itself could-be produced using chemicalmethods to synthesize the Flk-1 amino acid sequence in whole or in part.For example, peptides can be synthesized by solid phase techniques,cleaved from the resin, and purified by preparative high performanceliquid chromatography. (E.g., see Creighton, 1983, Proteins StructuresAnd Molecular Principles, W. H. Freeman and Co., N.Y. pp. 50-60). Thecomposition of the synthetic peptides may be confirmed by amino acidanalysis or sequencing (e.g., the Edman degradation procedure; seeCreighton, 1983, Proteins, Structures and Molecular Principles, W. H.Freeman and Co., N.Y., pp. 34-49).

5.2. Expression of Flk-1 Receptor and Generation of Cell Lines thatExpress Flk-1

In order to express a biologically active Flk-1, the nucleotide sequencecoding for Flk-1, or a functional equivalent as described in Section 5.1supra, is inserted into an appropriate expression vector, i.e., a vectorwhich contains the necessary elements for the transcription andtranslation of the inserted coding sequence. The Flk-1 gene products aswell as host cells or cell lines transfected or transformed withrecombinant Flk-1 expression vectors can be used for a variety ofpurposes. These include but are not limited to generating antibodies(i.e., monoclonal or polyclonal) that bind to the receptor, includingthose that competitively inhibit binding of VEGF and “neutralize”activity of Flk-1 and the screening and selection of VEGF analogs ordrugs-that act via the Flk-1 receptor; etc.

5.2.1. Expression Systems

Methods which are well known to those skilled in the art can be used toconstruct expression vectors containing the Flk-1 coding sequence andappropriate transcriptional/translational control signals. These methodsinclude in vitro recombinant DNA techniques, synthetic techniques and invivo recombination/genetic recombination. See, for example, thetechniques described in Maniatis et al., 1989, Molecular Cloning ALaboratory Manual, Cold Spring Harbor Laboratory, N.Y. and Ausubel etal., 1989, Current Protocols in Molecular Biology, Greene PublishingAssociates and Wiley Interscience, N.Y.

A variety of host-expression vector systems may be utilized to expressthe Flk-1 coding sequence. These include but are not limited tomicroorganisms such as bacteria transformed with recombinantbacteriophage DNA, plasmid DNA or cosmid DNA expression vectorscontaining the Flk-1 coding sequence; yeast transformed with recombinantyeast expression vectors containing the Flk-1 coding sequence; insectcell systems infected with recombinant virus expression vectors (e.g.,baculovirus) containing the Flk-1 coding sequence; plant cell systemsinfected with recombinant virus expression vectors (e.g., cauliflowermosaic virus, CaMV; tobacco mosaic virus, TMV) or transformed withrecombinant plasmid expression vectors (e.g., Ti plasmid) containing theFlk-1 coding sequence; or animal cell systems infected with recombinantvirus expression vectors (e.g., adenovirus, vaccinia virus) includingcell lines engineered to contain multiple copies of the Flk-1 DNA eitherstably amplified (CHO/dhfr) or unstably amplified in double-minutechromosomes (e.g., murine cell lines).

The expression elements of these systems vary in their strength andspecificities. Depending on the host/vector system utilized, any of anumber of suitable transcription and translation elements, includingconstitutive and inducible promoters, may be used in the expressionvector. For example, when cloning in bacterial systems, induciblepromoters such as pL of bacteriophage λ, plac, ptrp, ptac (ptrp-lachybrid promoter) and the like may be used; when cloning in insect cellsystems, promoters such as the baculovirus polyhedrin promoter may beused; when cloning in plant cell systems, promoters derived from thegenome of plant cells (e.g., heat shock promoters; the promoter for thesmall subunit of RUBISCO; the promoter for the chlorophyll a/b bindingprotein) or from plant viruses (e.g., the 35S RNA promoter of CaMV; thecoat protein promoter of TMV) may be used; when cloning in mammaliancell systems, promoters derived from the genome of mammalian cells(e.g., metallothionein promoter) or from mammalian viruses (e.g., theadenovirus late promoter; the vaccinia virus 7.5K promoter) may be used;when generating cell lines that contain multiple copies of the Flk-1 DNASV40-, BPV- and EBV-based vectors may be used with an appropriateselectable marker.

In bacterial systems a number of expression vectors may beadvantageously selected depending upon the use intended for the Flk-1expressed. For example, when large quantities of Flk-1 are to beproduced for the generation of antibodies or to screen peptidelibraries, vectors which direct the expression of high levels of fusionprotein products that are readily purified may be desirable. Suchvectors include but are not limited to the E. coli expression vectorpUR278 (Ruther et al., 1983, EMBO J. 2:1791), in which the Flk-1 codingsequence may be ligated into the vector in frame with the lac Z codingregion so that a hybrid AS-lac Z protein is produced; pIN vectors(Inouye & Inouye, 1985, Nucleic acids Res. 13:3101-3109; Van Heeke &Schuster, 1989, J. Biol. Chem. 264:5503-5509); and the like. PGEXvectors may also be used to express foreign polypeptides as fusionproteins with glutathione S-transferase (GST). In general, such fusionproteins are soluble and can easily be purified from lysed cells byadsorption to glutathione-agarose beads followed by elution in thepresence of free glutathione. The pGEX vectors are designed to includethrombin or factor Xa protease cleavage sites so that the clonedpolypeptide of interest can be released from the GST moiety.

In yeast, a number of vectors containing constitutive or induciblepromoters may be used. For a review see, Current Protocols in MolecularBiology, Vol. 2, 1988, Ed. Ausubel et al., Greene Publish. Assoc. &Wiley Interscience, Ch. 13; Grant et al., 1987, Expression and SecretionVectors for Yeast, in Methods in Enzymology, Eds. Wu & Grossman, 1987,Acad. Press, N.Y., Vol. 153, pp. 516-544; Glover, 1986, DNA Cloning,Vol. II, IRL Press, Wash., D.C., Ch. 3; and Bitter, 1987, HeterologousGene Expression in Yeast, Methods in Enzymology, Eds. Berger & Kimmel,Acad. Press, N.Y., Vol. 152, pp. 673-684; and The Molecular Biology ofthe Yeast Saccharomyces, 1982, Eds. Strathern et al., Cold Spring HarborPress, Vols. I and II.

In cases where plant expression vectors are used, the expression of theFlk-1 coding sequence may be driven by any of a number of promoters. Forexample, viral promoters such as the 35S RNA and 19S RNA promoters ofCaMV (Brisson et al., 1984, Nature 310:511-514), or the coat proteinpromoter of TMV (Takamatsu et al., 1987, EMBO J. 6:307-311) may be used;alternatively, plant promoters such as the small subunit of RUBISCO(Coruzzi et al., 1984, EMBO J. 3:1671-1680; Broglie et al., 1984,Science 224:838-843); or heat shock promoters, e.g., soybean hsp17.5-Eor hsp17.3-B (Gurley et al., 1986, Mol. Cell. Biol. 6:559-565) may beused. These constructs can be introduced into plant cells using Tiplasmids, R1 plasmids, plant virus vectors, direct DNA transformation,microinjection, electroporation, etc. For reviews of such techniquessee, for example, Weissbach & Weissbach, 1988, Methods for PlantMolecular Biology, Academic Press, NY, Section VIII, pp. 421-463; andGrierson & Corey, 1988, Plant Molecular Biology, 2d Ed., Blackie,London, Ch. 7-9.

An alternative expression system which could be used to express Flk-1 isan insect system. In one such system, Autographa californica nuclearpolyhidrosis virus (AcNPV) is used as a vector to express foreign genes.The virus grows in Spodoptera frugiperda cells. The Flk-1 codingsequence may be cloned into non-essential regions (for example thepolyhedrin gene) of the virus and placed under control of an AcNPVpromoter (for example the polyhedrin promoter). Successful insertion ofthe Flk-1 coding sequence will result in inactivation of the polyhedringene and production of non-occluded recombinant virus (i.e., viruslacking the proteinaceous coat coded for by the polyhedrin gene). Theserecombinant viruses are then used to infect Spodoptera frugiperda cellsin which the inserted gene is expressed. (E.g., see Smith et al., 1983,J. Viol. 46:584; Smith, U.S. Pat. No. 4,215,051).

In mammalian host cells, a number of viral based expression systems maybe utilized. In cases where an adenovirus is used as an expressionvector, the Flk-1 coding sequence may be ligated to an adenovirustranscription/translation control complex, e.g., the late promoter andtripartite leader sequence. This chimeric gene may then be inserted inthe adenovirus genome by in vitro or in vivo recombination. Insertion ina non-essential region of the viral genome (e.g., region E1 or E3) willresult in a recombinant virus that is viable and capable of expressingFlk-1 in infected hosts. (E.g., See Logan & Shenk, 1984, Proc. Natl.Acad. Sci. (USA) 81:3655-3659). Alternatively, the vaccinia 7.5Kpromoter may be used. (See, e.g., Mackett et al., 1982, Proc. Natl.Acad. Sci. (USA) 79:7415-7419; Mackett et al., 1984, J. Virol.49:857-864; Panicali et al., 1982, Proc. Natl. Acad. Sci. 79:4927-4931).

Specific initiation signals may also be required for efficienttranslation of inserted Flk-1 coding sequences. These signals includethe ATG initiation codon and adjacent sequences. In cases where theentire Flk-1 gene, including its own initiation codon and adjacentsequences, is inserted into the appropriate expression vector, noadditional translational control signals may be needed. However, incases where only a portion of the Flk-1 coding sequence is inserted,exogenous translational control signals, including the ATG initiationcodon, must be provided. Furthermore, the initiation codon must be inphase with the reading frame of the Flk-1 coding sequence to ensuretranslation of the entire insert. These exogenous translational controlsignals and initiation codons can be of a variety of origins, bothnatural and synthetic. The efficiency of expression may be enhanced bythe inclusion of appropriate transcription enhancer elements,transcription terminators, etc. (see Bittner et al., 1987, Methods inEnzymol. 153:516-544).

In addition, a host cell strain may be chosen which modulates theexpression of the inserted sequences, or modifies and processes the geneproduct in the specific fashion desired. Such modifications (e.g.,glycosylation) and processing (e.g., cleavage) of protein products maybe important for the function of the protein. Different host cells havecharacteristic and specific mechanisms for the post-translationalprocessing and modification of proteins. Appropriate cells lines or hostsystems can be chosen to ensure the correct modification and processingof the foreign protein expressed. To this end, eukaryotic host cellswhich possess the cellular machinery for proper processing of theprimary transcript, glycosylation, and phosphorylation of the geneproduct may be used. Such mammalian host cells include but are notlimited to CHO, VERO, BHK, HeLa, COS, MDCK, 293, WI38, etc.

For long-term, high-yield production of recombinant proteins, stableexpression is preferred. For example, cell lines which stably expressthe Flk-1 may be engineered. Rather than using expression vectors whichcontain viral origins of replication, host cells can be transformed withthe Flk-1 DNA controlled by appropriate expression control elements(e.g., promoter, enhancer, sequences, transcription terminators,polyadenylation sites, etc.), and a selectable marker. Following theintroduction of foreign DNA, engineered cells may be allowed to grow for1-2 days in an enriched media, and then are-switched to a selectivemedia. The selectable marker in the recombinant plasmid confersresistance to the selection and allows cells to stably integrate theplasmid into their chromosomes and grow to form foci which in turn canbe cloned and expanded into cell lines. This method may advantageouslybe used to engineer cell lines which express the Flk-1 on the cellsurface, and which respond to VEGF mediated signal transduction. Suchengineered cell lines are particularly useful in screening VEGF analogs.

A number of selection systems may be used, including but not limited tothe herpes simplex virus thymidine kinase (Wigler, et al., 1977, Cell11:223), hypoxanthine-guanine phosphoribosyltransferase (Szybalska &Szybalski, 1962, Proc. Natl. Acad. Sci. USA 48:2026), and adeninephosphoribosyltransferase (Lowy, et al., 1980, Cell 22:817) genes can beemployed in tk⁻, hgprt⁻ or aprt⁻ cells, respectively. Also,antimetabolite resistance can be used as the basis of selection fordhfr, which confers resistance to methotrexate (Wigler, et al., 1980,Natl. Acad. Sci. USA 77:3567; O'Hare, et al., 1981, Proc. Natl. Acad.Sci. USA 78:1527); gpt, which confers resistance to mycophenolic acid(Mulligan & Berg, 1981), Proc. Natl. Acad. Sci. USA 78:2072); neo, whichconfers resistance to the aminoglycoside G-418 (Colberre-Garapin, etal., 1981, J. Mol. Biol. 150:1); and hygro, which confers resistance tohygromycin (Santerre, et al., 1984, Gene 30:147) genes. Recently,additional selectable genes have been described, namely trpB, whichallows cells to utilize indole in place of tryptophan; hisD, whichallows cells to utilize histinol in place of histidine (Hartman &Mulligan, 1988, Proc. Natl. Acad. Sci. USA 85:8047); and ODC (ornithinedecarboxylase) which confers resistance to the ornithine decarboxylaseinhibitor, 2-(difluoromethyl)-DL-ornithine, DFMO (McConlogue L., 1987,In: Current Communications in Molecular Biology, Cold Spring HarborLaboratory ed.).

5.2.2. Identification of Transfectants or Transformants that Express theFlk-1

The host cells which contain the coding sequence and which express thebiologically active gene product may be identified by at least fourgeneral approaches; (a) DNA-DNA or DNA-RNA hybridization; (b) thepresence or absence of “marker” gene functions; (c) assessing the levelof transcription as measured by the expression of Flk-1 mRNA transcriptsin the host cell; and (d) detection of the gene product as measured byimmunoassay or by its biological activity.

In the first approach, the presence of the Flk-1 coding sequenceinserted in the expression vector can be detected by DNA-DNA or DNA-RNAhybridization using probes comprising nucleotide sequences that arehomologous to the Flk-1 coding sequence, respectively, or portions orderivatives thereof.

In the second approach, the recombinant expression vector/host systemcan be identified and selected based upon the presence or absence ofcertain “marker” gene functions (e.g., thymidine kinase activity,resistance to antibiotics, resistance to methotrexate, transformationphenotype, occlusion body formation in baculovirus, etc.). For example,if the Flk-1 coding sequence is inserted within a marker gene sequenceof the vector, recombinants containing the Flk-1 coding sequence can beidentified by the absence of the marker gene function. Alternatively, amarker gene can be placed in tandem with the Flk-1 sequence under thecontrol of the same or different promoter used to control the expressionof the Flk-1 coding sequence. Expression of the marker in response toinduction or selection indicates expression of the Flk-1 codingsequence.

In the third approach, transcriptional activity for the Flk-1 codingregion can be assessed by hybridization assays. For example, RNA can beisolated and analyzed by Northern blot using a probe homologous to theFlk-1 coding sequence or particular portions thereof. Alternatively,total nucleic acids of the host cell may be extracted and assayed forhybridization to such probes.

In the fourth approach, the expression of the Flk-1 protein product canbe assessed immunologically, for example by Western blots, immunoassayssuch as radioimmuno-precipitation, enzyme-linked immunoassays and thelike. The ultimate test of the success of the expression system,however, involves the detection of the biologically active Flk-1 geneproduct. A number of assays can be used to detect receptor activityincluding but not limited to VEGF binding assays; and VEGF biologicalassays using engineered cell lines as the test substrate.

5.3. Uses of the Flk-1 Receptor and Engineered Cell Lines

Angiogenesis, the growth of new blood capillary vessels, is required fora number of physiological processes ranging from wound healing, tissueand organ regeneration, placental formation after pregnancy andembryonic development. Abnormal proliferation of blood vessels is animportant component of a variety of diseases such as rheumatoidarthritis, retinopathies, and psoriasis. Angiogenesis is also animportant factor in the growth and metastatic activity of solid tumorsthat rely on vascularization. Therefore, inhibitors of angiogenesis maybe used therapeutically for the treatment of diseases resulting from oraccompanied by abnormal growth of blood vessels and for treatments ofmalignancies involving growth and spread of solid tumors.

In an embodiment of the invention the Flk-1 receptor and/or cell linesthat express the Flk-1 receptor may be used to screen for antibodies,peptides, organic molecules or other ligands that act as agonists orantagonists of angiogenesis or vasculogenesis mediated by the Flk-1receptor. For example, anti-Flk-1 antibodies capable of neutralizing theactivity of VEGF, may be used to inhibit Flk-1 function. Additionally,anti-Flk-1 antibodies which mimic VEGF activity may be selected for usesin wound healing. Alternatively, screening of peptide libraries ororganic compounds with recombinantly expressed soluble Flk-1 protein orcell lines expressing Flk-1 protein may be useful for identification oftherapeutic molecules that function by inhibiting the biologicalactivity of Flk-1.

In an embodiment of the invention, engineered cell lines which expressthe entire Flk-1 coding region or its ligand binding domain may beutilized to screen and identify VEGF antagonists as well as agonists.Synthetic compounds, natural products, and other sources of potentiallybiologically active materials can be screened in a number of ways. Theability of a test compound to inhibit binding of VEGF to Flk-1 may bemeasured using standard receptor binding techniques, such as thosedescribed in Section 6.1.9., or using any of the compound screeningassays described in Section 5.3.2. and 6.1.14. The ability of agents toprevent or mimic, the effect of VEGF binding on signal transductionresponses on Flk-1 expressing cells may be measured. For example,responses such as activation of Flk-1 kinase activity, modulation ofsecond messenger production or changes in cellular metabolism may bemonitored. These assays may be performed using conventional techniquesdeveloped for these purposes.

The ability of a test compound to modulate signal transduction throughthe VEGF-Flk-1 system may also be measured in vivo, in models such asthose described in Section 6.1.12. and 6.1.13. The ability of agents toprevent the effect of VEGF binding on signal transduction responses ofFlk-1 expressing cells may be measured. For example, responses such asinhibition of angiogenesis, inhibition of the development of solidtumors, or reduction of solid tumor size may be monitored.

5.3.1. Screening of Peptide Library with Flk-1 Protein or EngineeredCell Lines

Random peptide libraries consisting of all possible combinations ofamino acids attached to a solid phase support may be used to identifypeptides that are able to bind to the ligand binding site of a givenreceptor or other functional domains of a receptor such as kinasedomains (Lam, K. S. et al., 1991, Nature 354:82-84). The screening ofpeptide libraries may have therapeutic value in the discovery ofpharmaceutical agents that act to inhibit the biological activity ofreceptors through their interactions with the given receptor.

Identification of molecules that are able to bind to the Flk-1 may beaccomplished by screening a peptide library with recombinant solubleFlk-1 protein. Methods for expression and purification of Flk-1 aredescribed in Section 5.2.1 and may be used to express recombinant fulllength Flk-1 or fragments of Flk-1 depending on the functional domainsof interest. For example, the kinase and extracellular ligand bindingdomains of Flk-1 may be separately expressed and used to screen peptidelibraries.

To identify and isolate the peptide/solid phase support that interactsand forms a complex with Flk-1, it is necessary to label or “tag” theFlk-1 molecule. The Flk-1 protein may be conjugated to enzymes such asalkaline phosphatase or horseradish peroxidase or to other reagents suchas fluorescent labels which may include fluorescein isothyiocynate(FITC), phycoerythrin (PE) or rhodamine. Conjugation of any given label,to Flk-1, may be performed using techniques that are routine in the art.Alternatively, Flk-1 expression vectors may be engineered to express achimeric Flk-1 protein containing an epitope for which a commerciallyavailable antibody exist. The epitope specific antibody may be taggedusing methods well known in the art including labeling with enzymes,fluorescent dyes or colored or magnetic beads.

The “tagged” Flk-1 conjugate is incubated with the random peptidelibrary for 30 minutes to one hour at 22° C. to allow complex formationbetween Flk-1 and peptide species within the library. The library isthen washed to remove any unbound Flk-1 protein. If Flk-1 has beenconjugated to alkaline phosphatase or horseradish peroxidase the wholelibrary is poured into a petri dish containing a substrates for eitheralkaline phosphatase or peroxidase, for example,5-bromo-4-chloro-3-indoyl phosphate (BCIP) or 3,3′,4,4″-diamnobenzidine(DAB), respectively. After incubating for several minutes, thepeptide/solid phase-Flk-1 complex changes color, and can be easilyidentified and isolated physically under a dissecting microscope with amicromanipulator. If a fluorescent tagged Flk-1 molecule has been used,complexes may be isolated by fluorescent activated sorting. If achimeric Flk-1 protein expressing a heterologous epitope has been used,detection of the peptide/Flk-1 complex may be accomplished by using alabeled epitope specific antibody. Once isolated, the identity of thepeptide attached to the solid phase support may be determined by peptidesequencing.

In addition to using soluble Flk-1 molecules, in another embodiment, itis possible to detect peptides that bind to cell surface receptors usingintact cells. The use of intact cells is preferred for use withreceptors that are multi-subunits or labile or with receptors thatrequire the lipid domain of the cell membrane to be functional. Methodsfor generating cell lines expressing Flk-1 are described in Sections5.2.1. and 5.2.2. The cells used in this technique may be either live orfixed cells. The cells will be incubated with the random peptide libraryand will bind to certain peptides in the library to form a “rosette”between the target cells and the relevant solid phase support/peptide.The rosette can thereafter be isolated by differential centrifugation orremoved physically under a dissecting microscope.

As an alternative to whole cell assays for membrane bound receptors orreceptors that require the lipid domain of the cell membrane to befunctional, the receptor molecules can be reconstituted into liposomeswhere label or “tag” can be attached.

5.3.2. Screening of Organic Compounds with Flk-1 Protein or EngineeredCell Lines

Cell lines that express Flk-1 may be used to screen for molecules thatmodulate Flk-1 receptor activity or signal transduction. Such moleculesmay include small organic or inorganic compounds, or other moleculesthat modulate Flk-1 receptor activity or that promote or prevent theformation of Flk-1/VEGF complex. Synthetic compounds, natural products,and other sources of potentially biologically active materials can bescreened in a number of ways.

The ability of a test molecule to interfere with VEGF-Flk-1 bindingand/or Flk-1 receptor signal may be measured using standard biochemicaltechniques. Other responses such as activation or suppression ofcatalytic activity, phosphorylation or dephosphorylation of otherproteins, activation or modulation of second messenger production,changes in cellular ion levels, association, dissociation ortranslocation of signalling molecules, or transcription or translationof specific genes may also be monitored. These assays may be performedusing conventional techniques developed for these purposes in the courseof screening.

Ligand binding to its cellular receptor may, via signal transductionpathways, affect a variety of cellular processes. Cellular processesunder the control of the Flk-1/VEGF signalling pathway may include, butare not limited to, normal cellular functions, proliferation,differentiation, maintenance of cell shape, and adhesion, in addition toabnormal or potentially deleterious processes such as unregulated cellproliferation, loss of contact inhibition, blocking of differentiationor cell death. The qualitative or quantitative observation andmeasurement of any of the described cellular processes by techniquesknown in the art may be advantageously used as a means of scoring forsignal transduction in the course of screening.

Various embodiments are described below for screening, identificationand evaluation of compounds that interact with the Flk-1 receptor, whichcompounds may affect various cellular processes under the control of theFlk/VEGF receptor signalling pathway.

The present invention includes a method for identifying a compound whichis capable of modulating signal transduction, comprising:

-   -   (a) contacting the compound with Flk-1, or a functional        derivative thereof, in pure or semi-pure form, in a membrane        preparation, or in a whole live or fixed cell;    -   (b) incubating the mixture of step (a) in the presence of VEGF,        for an interval sufficient for the compound to stimulate or        inhibit the signal transduction;    -   (c) measuring the signal transduction;    -   (d) comparing the signal transduction activity to that of Flk-1,        incubated without the compound, thereby determining whether the        compound stimulates or inhibits signal transduction.

Flk-1, or functional derivatives thereof, useful in identifyingcompounds capable of modulating signal transduction may have, forexample, amino acid deletions and/or insertions and/or substitutions aslong as they retain significant signal transducing capacity. Afunctional derivative of Flk-1 may be prepared from a naturallyoccurring or recombinantly expressed Flk-1 by proteolytic cleavagefollowed by conventional purification procedures known to those skilledin the art. Alternatively, the functional derivative may be produced byrecombinant DNA technology by expressing parts of Flk-1 which includethe functional domain in suitable cells. Functional derivatives may alsobe chemically synthesized. Cells expressing Flk-1 may be used as asource of Flk-1, crude or purified, or in a membrane preparation, fortesting in these assays. Alternatively, whole live or fixed cells may beused directly in those assays.

Flk-1 signal transduction activity may be measured by standardbiochemical techniques or by monitoring the cellular processescontrolled by the signal. To assess modulation of kinase activity, thetest molecule is added to a reaction mixture containing Flk-1 and asubstrate test. To assess modulation of kinase activity of the Flk-1receptor, the test molecule is added to a reaction mixture containingthe Flk-1 receptor. The kinase reaction is then initiated with theaddition of VEGF and ATP. An immunoassay is performed on the kinasereaction to detect the presence or absence of the phosphorylatedtyrosine residues on the substrate or to detect phosphorylated tyrosineresidues on autophosphorylated Flk-1, and results are compared to thoseobtained for controls i.e., reaction mixtures not exposed to the testmolecule. The immunoassay used to detect the phosphorylated substrate inthe cell lysate or the in vitro reaction mixture may be carried out withan anti-phosphotyrosine antibody.

The invention further provides for a method of screening compounds that,upon interacting with Flk-1, elicit or trigger a signal mimicking theaction of VEGF binding to the Flk-1 receptor. Signal transduction ismimicked if the cellular processes under the control of the signallingpathway are affected in a way similar to that caused by ligand binding.Such compounds may be naturally occurring or synthetically producedmolecules that activate the Flk-1 receptor.

The invention also includes a method whereby a molecule capable ofbinding to Flk-1 in a chemical or biological preparation may beidentified comprising:

-   -   (a) immobilizing Flk-1, or functional fragments thereof, to a        solid phase matrix;    -   (b) contacting the chemical or biological preparation with the        solid phase matrix produced in step (a), for an interval        sufficient to allow the compound to bind;    -   (c) washing away any unbound material from the solid phase        matrix;    -   (d) detecting the presence of the compound bound to the solid        phase,        thereby identifying the compound.        The above method may further include the step of:    -   (e) eluting the bound compound from the solid phase matrix,        thereby isolating the compound.

The term “compound capable of binding to Flk-1” refers to a naturallyoccurring or synthetically produced molecule which interacts with Flk-1.Such a compound may directly or indirectly modulate Flk-1 signaltransduction and may include molecules that are natively associated withthe intracellular domain of Flk-1 inside a cell. Examples of suchcompounds are (i) a natural substrate of Flk-1; (ii) a naturallyoccurring molecule which is part of the signalling complex; and/or anaturally occurring signalling molecule produced by other cell types.

5.3.3. Antibody Production and Screening

Various procedures known in the art may be used for the production ofantibodies to epitopes of the recombinantly produced Flk-1 receptor.Such antibodies include but are not limited to polyclonal, monoclonal,chimeric, single chain, Fab fragments and fragments produced by an Fabexpression library. Neutralizing antibodies i.e., those which competefor the VEGF binding site of the receptor are especially preferred fordiagnostics and therapeutics.

Monoclonal antibodies that bind Flk-1 may be radioactively labeledallowing one to follow their location and distribution in the body afterinjection. Radioactivity tagged antibodies may be used as a non-invasivediagnostic tool for imaging de novo vascularization associated with anumber of diseases including rheumatoid arthritis, macular degeneration,and formation of tumors and metastases.

Immunotoxins may also be designed which target cytotoxic agents tospecific sites in the body. For example, high affinity Flk-1 specificmonoclonal antibodies may be covalently complexed to bacterial or planttoxins, such as diptheria toxin, abrin or ricin. A general method ofpreparation of antibody/hybrid molecules may involve use ofthiol-crosslinking reagents such as SPDP, which attack the primary aminogroups on the antibody and by disulfide exchange, attach the toxin tothe antibody. The hybrid antibodies may be used to specificallyeliminate Flk-1 expressing endothelial cells.

For the production of antibodies, various host animals may be immunizedby injection with the Flk-1 protein including but not limited torabbits, mice, rats, etc. Various adjuvants may be used to increase theimmunological response, depending on the host species, including but notlimited to Freund's (complete and incomplete), mineral gels such asaluminum hydroxide, surface active substances such as lysolecithin,pluronic polyols, polyanions, peptides, oil emulsions, keyhole limpethemocyanin, dinitrophenol, and potentially useful human adjuvants suchas BCG (bacille Calmette-Guerin) and Corynebacterium parvum.

Monoclonal antibodies to Flk-1 may be prepared by using any techniquewhich provides for the production of antibody molecules by continuouscell lines in culture. These include but are not limited to thehybridoma technique originally described by Kohler and Milstein,(Nature, 1975, 256:495-497), the human B-cell hybridoma technique(Kosbor et al., 1983, Immunology Today, 4:72; Cote et al., 1983, Proc.Natl. Acad. Sci., 80:2026-2030) and the EBV-hybridoma technique (Cole etal., 1985, Monoclonal Antibodies and Cancer Therapy, Alan R. Liss, Inc.,pp. 77-96). In addition, techniques developed for the production of“chimeric antibodies” (Morrison et al., 1984, Proc. Natl. Acad. Sci.,81:6851-6855; Neuberger et al., 1984, Nature, 312:604-608; Takeda etal., 1985, Nature, 314:452-454) by splicing the genes from a mouseantibody molecule of appropriate antigen specificity together with genesfrom a human antibody molecule of appropriate biological activity can beused. Alternatively, techniques described for the production of singlechain antibodies (U.S. Pat. No. 4,946,778) can be adapted to produceFlk-1-specific single chain antibodies.

Antibody fragments which contain specific binding sites of Flk-1 may begenerated by known techniques. For example, such fragments include butare not limited to: the F(ab′)₂ fragments which can be produced bypepsin digestion of the antibody molecule and the Fab fragments whichcan be generated by reducing the disulfide bridges of the F(ab′)₂fragments. Alternatively, Fab expression libraries may be constructed(Huse et al., 1989, Science, 246:1275-1281) to allow rapid and easyidentification of monoclonal Fab fragments with the desired specificityto Flk-1.

5.4. Uses of Flk-1 Coding Sequence

The Flk-1 coding sequence may be used for diagnostic purposes fordetection of Flk-1 expression. Included in the scope of the inventionare oligoribonucleotide sequences, that include antisense RNA and DNAmolecules and ribozymes that function to inhibit translation of Flk-1.In addition, mutated forms of Flk-1, having a dominant negative effect,may be expressed in targeted cell populations to inhibit the activity ofendogenously expressed wild-type Flk-1.

5.4.1. Use of Flk-1 Coding Sequence in Diagnostics and Therapeutics

The Flk-1 DNA may have a number of uses for the diagnosis of diseasesresulting from aberrant expression of Flk-1. For example, the Flk-1 DNAsequence may be used in hybridization assays of biopsies or autopsies todiagnose abnormalities of Flk-1 expression; e.g., Southern or Northernanalysis, including in situ hybridization assays.

The Flk-1 cDNA may be used as a probe to detect the expression of theFlk-1 mRNA. In a specific example described herein, the expression ofFlk-1 mRNA in mouse embryos of different developmental stages wasanalyzed. Northern blot analysis indicated abundant expression of amajor 5.5 kb mRNA between day 9.5 and day 18.5, with apparent declinetowards the end of gestation (FIG. 2A). In post-natal day 4-8 braincapillaries Flk-1 mRNA was found to be highly enriched compared to totalbrain RNA (FIG. 2B), suggesting a role for Flk-1 in endothelial cellproliferation.

To obtain more detailed information about the expression of Flk-1 duringembryonic development and during the early stages of vasculardevelopment in situ hybridization experiments were performed asdescribed in Section 6.1.4. In situ hybridizations demonstrated thatFlk-1 expression in vivo during embryonic mouse development is largelyrestricted to endothelial cells and their precursors (FIGS. 3A, 3B, 3Cand FIGS. 4A, 4B, 4C, 4D and 4E). Flk-1 is expressed in endothelialcells during physiological processes that are characterized byendothelial cell proliferation and the temporal and spatial expressionpattern found in the embryonic brain correlate precisely with thedevelopment of the neural vascular system as described by Bar (1980).Vascular sprouts originating in the perineural plexus grow radially intothe neuroectoderm and branch there and these sprouts were found toexpress high amounts of Flk-1 mRNA (FIGS. 5A, 5B, 5C and 5D). In theearly postnatal stages, endothelial cell proliferation is still evidentand Flk-1 is expressed, whereas in the adult organism, after completionof the vascularization process, the decline in endothelial cellproliferation parallels a decrease in Flk-1 expression.

Also within the scope of the invention are oligo-ribonucleotidesequences, that include anti-sense RNA and DNA molecules and ribozymesthat function to inhibit the translation of Flk-1 mRNA. Anti-sense RNAand DNA molecules act to directly block the translation of mRNA bybinding to targeted mRNA and preventing protein translation. In regardto antisense DNA, oligodeoxyribonucleotides derived from the translationinitiation site, e.g., between −10 and +10 regions of the Flk-1nucleotide sequence, are preferred.

Ribozymes are enzymatic RNA molecules capable of catalyzing the specificcleavage of RNA. The mechanism of ribozyme action involves sequencespecific hybridization of the ribozyme molecule to complementary targetRNA, followed by a endonucleolytic cleavage. Within the scope of theinvention are engineered hammerhead motif ribozyme molecules thatspecifically and efficiently catalyze endonucleolytic cleavage of Flk-1RNA sequences.

Specific ribozyme cleavage sites within any potential RNA target areinitially identified by scanning the target molecule for ribozymecleavage sites which include the following sequences, GUA, GUU and GUC.Once identified, short RNA sequences of between 15 and 20ribonucleotides corresponding to the region of the target genecontaining the cleavage site may be evaluated for predicted structuralfeatures such as secondary structure that may render the oligonucleotidesequence unsuitable. The suitability of candidate targets may also beevaluated by testing their accessibility to hybridization withcomplementary oligonucleotides, using ribonuclease protection assays.

Both anti-sense RNA and DNA molecules and ribozymes of the invention maybe prepared by any method known in the art for the synthesis of RNAmolecules. These include techniques for chemically synthesizingoligodeoxyribonucleotides well known in the art such as for examplesolid phase phosphoramidite chemical synthesis. Alternatively, RNAmolecules may be generated by in vitro and in vivo transcription of DNAsequences encoding the antisense RNA molecule. Such DNA sequences may beincorporated into a wide variety of vectors which incorporate suitableRNA polymerase promoters such as the T7 or SP6 polymerase promoters.Alternatively, antisense cDNA constructs that synthesize antisense RNAconstitutively or inducibly, depending on the promoter used, can beintroduced stably into cell lines.

Various modifications to the DNA molecules may be introduced as a meansof increasing intracellular stability and half-life. Possiblemodifications include but are not limited to the addition of flankingsequences of ribo- or deoxy-nucleotides to the 5′ and/or 3′ ends of themolecule or the use of phosphorothioate or 2′ O-methyl rather thanphosphodiesterase linkages within the oligodeoxyribonucleotide backbone.

5.4.2. Use of Dominant Negative Flk-1 Mutants in Gene Therapy

Receptor dimerization induced by ligands, is thought to provide anallosteric regulatory signal that functions to couple ligand binding tostimulation of kinase activity. Defective receptors can function asdominant negative mutations by suppressing the activation and responseof normal receptors through formation of heterodimers with wild typereceptors wherein such heterodimers are signalling incompetent.Defective receptors can be engineered into recombinant viral vectors andused in gene therapy in individuals that inappropriately express Flk-1.

The capability of Flk-1 TM to form signalling incompetent heterodimerswith the 180 kD wild type Flk-1 is demonstrated in Section 6.1.12. Thedominant-negative potential of Flk-1 TM used in gene therapy may bemeasured by examining its influence on the Flk-1/VEGF mitogenic responseor by measurement of suppression of Flk-1 transforming activity.

In an embodiment of the invention, mutant forms of the Flk-1 moleculehaving a dominant negative effect may be identified by expression inselected cells. Deletion or missense mutants of Flk-1 that retain theability to form dimers with wild type Flk-1 protein but cannot functionin signal transduction may be used to inhibit the biological activity ofthe endogenous wild type Flk-1. For example, the cytoplasmic kinasedomain of Flk-1 may be deleted resulting in a truncated Flk-1 moleculethat is still able to undergo dimerization with endogenous wild typereceptors but unable to transduce a signal.

Abnormal proliferation of blood vessels is an important component of avariety of pathogenic disorders such as rheumatoid arthritis,retinopathies and psoriasis. Uncontrolled angiogenesis is also animportant factor in the growth and metastases of solid tumors.Recombinant viruses may be engineered to express dominant negative formsof Flk-1 which may be used to inhibit the activity of the wild typeendogenous Flk-1. These viruses may be used therapeutically fortreatment of diseases resulting from aberrant expression or activity ofFlk-1.

Expression vectors derived from viruses such as retroviruses, vacciniavirus, adeno-associated virus, herpes viruses, or bovine papillomavirus, may be used for delivery of recombinant Flk-1 into the targetedcell population. Methods which are well known to those skilled in theart can be used to construct recombinant viral vectors containing Flk-1coding sequence. See, for example, the techniques described in Maniatiset al., 1989, Molecular Cloning A Laboratory Manual, Cold Spring HarborLaboratory, N.Y. and Ausubel et al., 1989, Current Protocols inMolecular Biology, Greene Publishing Associates and Wiley Interscience,N.Y. Alternatively, recombinant Flk-1 molecules can be reconstitutedinto liposomes for delivery to target cells.

In a specific embodiment of the invention, a deletion mutant of theFlk-1 receptor was engineered into a recombinant retroviral vector. Twoclonal isolates of Flk-1 TM, designated pLXSN Flk-1 TM cl.1 and pLXSNFlk-1 TM cl.3, contain a truncated Flk-1 receptor containing Flk-1 aminoacids 1 through 806 but lacking the 561 COOH-terminal amino acids of theintracellular kinase domain. These isolates retain transmembrane domainsequences and 23 residues of the cytoplasmic domain. To obtain virusproducing cell lines, PA37 cells were transfected with the recombinantvectors and, subsequently, conditioned media containing virus were usedto infect GPE cells.

To test whether expression of signaling-defective mutants inhibitsendogenous Flk-1 receptor activity, C6 rat gliobastoma cells (tumorcells) and mouse cells producing the recombinant retroviruses were mixedand injected subcutaneously into nude mice. Normally, injection of tumorcells into nude mice would result in proliferation of the tumor cellsand vascularization of the resulting tumor mass. Since Flk-1 is believedto be essential for formation of blood vessels, blocking Flk-1 activityby expression of a truncated receptor, might function to inhibitvascularization of the developing tumor and, thereby, inhibit itsgrowth. As illustrated in FIGS. 13 and 14, coinjection of virusproducing cells, expressing a truncated Flk-1 receptor, significantlyinhibits the growth of the tumor as compared to controls receiving onlytumor cells.

As illustrated in FIG. 15, a similar inhibitory effect on C6 gliomatumor growth was also observed when truncated Flk-1 receptor virusparticle-containing producer cells were injected five days afterimplantation of 10⁶ tumor cells, indicating that even established tumorsmay be suppressed by Flk-1 dominant-negative action. For glioblastoma, atumor with generally poor prognosis and resistance to all availabletherapies, retrovirus-mediated gene therapy may be advantageous, sincenon-mitotic brain tissues such as neurons, glia and quiescentendothelial cells would not be infected. Glioblastoma multiforme is themost common and most malignant tumor of astrocytic origin in humanadults and accounts for more than half of all primary brain tumors (See,for example, Cecil Textbook of Medicine, Wyngaarden, Smith, Bennett(eds) W B Saunders, p. 2220 (1992).

5.5. Use of Flk-1 Receptor or Ligands

Receptor/ligand interaction between Flk-1 and VEGF is believed to playan important role in the signalling system during vascularization andangiogenesis. Abnormal proliferation of blood vessels is an importantcomponent of a number of diseases.

Expression of Flk-1 RNA correlates with the development of the brain andwith endothelial cell proliferation suggesting that Flk-1 might be areceptor involved in mediation of signaling events in the neuralvascularization process. VEGF has been shown to be a mitogenic growthfactor known to act exclusively on endothelial cells (Ferrara, N. andHenzel, W. J., 1989, Biochem. Biophys. Res. Comm. 161:851-858).Cross-linking and ligand binding experiments were performed, asdescribed in Sections 6.1.9 and 6.1.10 respectively, to determinewhether VEGF is a ligand for Flk-1. The results indicate that Flk-1 isan authentic high affinity VEGF receptor (FIGS. 9A and 9B).

In one embodiment of the invention, ligands for Flk-1, the Flk-1receptor itself, or a fragment containing its VEGF binding site, couldbe administered in vivo to modulate angiogenesis and/or vasculogenesis.For example, administration of the Flk-1 receptor or a fragmentcontaining the VEGF binding site, could competitively bind to VEGF andinhibit its interaction with the native Flk-1 receptor in vivo toinhibit angiogenesis and/or vasculogenesis. Alternatively, ligands forFlk-1, including anti-Flk-1 antibodies or fragments thereof, may be usedto modulate angiogenesis and/or vasculogenesis. Agonists of VEGFactivity may be used to promote wound healing whereas antagonists ofVEGF activity may be used to inhibit tumor growth.

The particular peptides, proteins, organic compounds or antibodies thatmodulate Flk-1 receptor signal transduction can be administered to apatient either by itself, or in pharmaceutical compositions where it ismixed with suitable carriers or excipient(s).

Depending on the specific conditions being treated, these agents may beformulated and administered systemically or locally. Techniques forformulation and administration may be found in “Remington'sPharmaceutical Sciences,” Mack Publishing Co., Easton, Pa., latestedition. Suitable routes may, for example, include oral, rectal,transmucosal, or intestinal administration; parenteral delivery,including intramuscular, subcutaneous, intramedullary injections, aswell as intrathecal, direct intraventricular, intravenous,intraperitoneal, intranasal, or intraocular injections, or, in the caseof solid tumors, directly injected into a solid tumor. For injection,the agents of the invention may be formulated in aqueous solutions,preferably in physiologically compatible buffers such as Hanks'ssolution, Ringer's solution, or physiological saline buffer. Fortransmucosal administration, penetrants appropriate to the barrier to bepermeated are used in the formulation. Such penetrants are generallyknown in the art.

The compounds can be formulated readily using pharmaceuticallyacceptable carriers well known in the art into dosages suitable for oraladministration. Such carriers enable the compounds of the invention tobe formulated as tablets, pills, capsules, liquids, gels, syrups,slurries, suspensions and the like, for oral ingestion by a patient tobe treated.

Pharmaceutical compositions suitable for use in the present inventioninclude compositions wherein the active ingredients are contained in aneffective amount to achieve its intended purpose. Determination of theeffective amounts is well within the capability of those skilled in theart, especially in light of the detailed disclosure provided herein.

In addition to the active ingredients these pharmaceutical compositionsmay contain suitable pharmaceutically acceptable carriers comprisingexcipients and auxiliaries which facilitate processing of the activecompounds into preparations which can be used pharmaceutically. Thepreparations formulated for oral administration may be in the form oftablets, dragees, capsules, or solutions.

The pharmaceutical compositions of the present invention may bemanufactured in a manner that is itself known, e.g., by means ofconventional mixing, dissolving, granulating, dragee-making, levigating,emulsifying, encapsulating, entrapping or lyophilizing processes.

Pharmaceutical formulations for parenteral administration includeaqueous solutions of the active compounds in water-soluble form.Additionally, suspensions of the active compounds may be prepared asappropriate oily injection suspensions. Suitable lipophilic solvents orvehicles include fatty oils such as sesame oil, or synthetic fatty acidesters, such as ethyl oleate or triglycerides, or liposomes. Aqueousinjection suspensions may contain substances which increase theviscosity of the suspension, such as sodium carboxymethyl cellulose,sorbitol, or dextran. Optionally, the suspension may also containsuitable stabilizers or agents which increase the solubility of thecompounds to allow for the preparation of highly concentrated solutions.

Pharmaceutical preparations for oral use can be obtained by combiningthe active compounds with solid excipient, optionally grinding aresulting mixture, and processing the mixture of granules, after addingsuitable auxiliaries, if desired, to obtain tablets or dragee cores.Suitable excipients are, in particular, fillers such as sugars,including lactose, sucrose, mannitol, or sorbitol; cellulosepreparations such as, for example, maize starch, wheat starch, ricestarch, potato starch, gelatin, gum tragacanth, methyl cellulose,hydroxypropylmethyl-cellulose, sodium carboxymethylcellulose, and/orpolyvinylpyrrolidone (PVP). If desired, disintegrating agents may beadded, such as the cross-linked polyvinyl pyrrolidone, agar, or alginicacid or a salt thereof such as sodium alginate.

Dragee cores are provided with suitable coatings. For this purpose,concentrated sugar solutions may be used, which may optionally containgum arabic, talc, polyvinyl pyrrolidone, carbopol gel, polyethyleneglycol, and/or titanium dioxide, lacquer solutions, and suitable organicsolvents or solvent mixtures. Dyestuffs or pigments may be added to thetablets or dragee coatings for identification or to characterizedifferent combinations of active compound doses.

Pharmaceutical preparations which can be used orally include push-fitcapsules made of gelatin, as well as soft, sealed capsules made ofgelatin and a plasticizer, such as glycerol or sorbitol. The push-fitcapsules can contain the active ingredients in admixture with fillersuch as lactose, binders such as starches, and/or lubricants such astalc or magnesium stearate and, optionally, stabilizers. In softcapsules, the active compounds may be dissolved or suspended in suitableliquids, such as fatty oils, liquid paraffin, or liquid polyethyleneglycols. In addition, stabilizers may be added.

Compositions comprising a compound of the invention formulated in acompatible pharmaceutical carrier may be prepared, placed in anappropriate container, and labelled for treatment of an indicatedcondition. Suitable conditions indicated on the label may includetreatment of a tumor, such as a glioma or glioblastoma; and inhibitionof angiogenesis.

A preferred pharmaceutical carrier for hydrophobic compounds of theinvention is a cosolvent system comprising benzyl alcohol, a nonpolarsurfactant, a water-miscible organic polymer, and an aqueous phase. Apreferred cosolvent system is the VPD co-solvent system. VPD is asolution of 3% w/v benzyl alcohol, 8% w/v of the nonpolar surfactantpolysorbate 80, and 65% w/v polyethylene glycol 300, made up to volumein absolute ethanol. The VPD co-solvent system (VPD: 5W) consists of VPDdiluted 1:1 with a 5% dextrose in water solution. This co-solvent systemdissolves hydrophobic compounds well, and itself produces low toxicityupon systemic administration. Naturally, the proportions of a co-solventsystem may be varied considerably without destroying its solubility andtoxicity characteristics. Furthermore, the identity of the co-solventcomponents may be varied: for example, other low-toxicity nonpolarsurfactants may be used instead of polysorbate 80; the fraction size ofpolyethylene glycol may be varied; other biocompatible polymers mayreplace polyethylene glycol, e.g. polyvinyl pyrrolidone; and othersugars or polysaccharides may substitute for dextrose.

Alternatively, other delivery systems for hydrophobic pharmaceuticalcompounds may be employed. Liposomes and emulsions are well knownexamples of delivery vehicles or carriers for hydrophobic drugs. Certainorganic solvents such as DMSO also may be employed, although usually atthe cost of greater toxicity.

The pharmaceutical compositions also may comprise suitable solid or gelphase carriers or excipients. Examples of such carriers or excipientsinclude but are not limited to calcium carbonate, calcium phosphate,various sugars, starches, cellulose derivatives, gelatin, and polymerssuch as polyethylene glycols.

Many of the Flk-1 receptor modulating compounds of the invention may beprovided as salts with pharmaceutically compatible counterions.Pharmaceutically compatible salts may be formed with many acids,including but not limited to hydrochloric, sulfuric, acetic, lactic,tartaric, malic, succinic, etc. Salts tend to be more soluble in aqueousor other protonic solvents that are the corresponding free base forms.

For any compound used in the method of the invention, thetherapeutically effective dose can be estimated initially from cellculture assays. For example, a dose can be formulated in animal modelsto achieve a circulating concentration range that includes the IC50 asdetermined in cell culture (i.e., the concentration of the test compoundwhich achieves a half-maximal inhibition of the PTP activity). Suchinformation can be used to more accurately determine useful doses inhumans.

A therapeutically effective dose refers to that amount of the compoundthat results in amelioration of symptoms or a prolongation of survivalin a patient. Toxicity and therapeutic efficacy of such compounds can bedetermined by standard pharmaceutical procedures in cell cultures orexperimental animals, e.g., for determining the LD50 (the dose lethal to50% of the population) and the ED50 (the dose therapeutically effectivein 50% of the population). The dose ratio between toxic and therapeuticeffects is the therapeutic index and it can be expressed as the ratioLD50/ED50. Compounds which exhibit large therapeutic indices arepreferred. The data obtained from these cell culture assays and animalstudies can be used in formulating a range of dosage for use in human.The dosage of such compounds lies preferably within a range ofcirculating concentrations that include the ED50 with little or notoxicity. The dosage may vary within this range depending upon thedosage form employed and the route of administration utilized. The exactformulation, route of administration and dosage can be chosen by theindividual physician in view of the patient's condition. (See e.g. Finglet al., 1975, in “The Pharmacological Basis of Therapeutics”, Ch. 1 p1).

Dosage amount and interval may be adjusted individually to provideplasma levels of the active moiety which are sufficient to maintain theFlk-1 receptor-inhibitory effects. Usual patient dosages for systemicadministration range from 1-2000 mg/day, commonly from 1-250 mg/day, andtypically from 10-150 mg/day. Stated in terms of patient body weight,usual dosages range from 0.02-25 mg/kg/day, commonly from 0.02-3mg/kg/day, typically from 0.2-1.5 mg/kg/day. Stated in terms of patientbody surface areas, usual dosages range from 0.5-1200 mg/m²/day,commonly from 0.5-150 mg/m²/day, typically from 5-100 mg/m²/day.

Dosage amount and interval may be adjusted individually to provideplasma levels of the active moiety which are sufficient to maintain theFlk-1 receptor-inhibitory effects. Usual average plasma levels should bemaintained within 50-5000 μg/ml, commonly 50-1000 μg/ml, and typically100-500 μg/ml.

Alternately, one may administer the compound in a local rather thansystemic manner, for example, via injection of the compound directlyinto a tumor, often in a depot or sustained release formulation.

Furthermore, one may administer the drug in a targeted drug deliverysystem, for example, in a liposome coated with tumor-specific antibody.The liposomes will be targeted to and taken up selectively by the tumor.

In cases of local administration or selective uptake, the effectivelocal concentration of the drug may not be related to plasmaconcentration.

6. EXAMPLE Cloning and Expression Patterns of Flk-1, a High AffinityReceptor for VEGF

The subsection below describes the cloning and characterization of theFlk-1 cDNA clone. Northern blot and in situ hybridization analysesindicate that Flk-1 is expressed in endothelial cells. Cross-linking andligand binding experiments further indicate that Flk-1 is a highaffinity receptor for VEGF.

6.1. Materials and Methods

6.1.1. cDNA Cloning of Flk-1

DNA extracted from λgt10 cDNA library of day 8.5 mouse embryos (Fahrneret al., 1987, EMBO. J. 6:1497-1508) was used as template for polymerasechain reaction (PCR; Saiki, R. K. et al., 1985 Science 230:1350-1354).In an independent approach cDNA of capillary endothelial cells that hadbeen isolated from the brain of postnatal day 4-8 mice was used foramplification (Risau, W., 1990 In: development of the Vascular System.Issues Biomed. Basel Karger 58-68 and Schnürch et al., unpublished)Degenerated primers were designed on the basis of high amino acidhomologies within the kinase domain shared by all RTKs (Wilks, A. F.,1989, Proc. Natl. Acad. Sci. U.S.A. 86:1603-1607).

Full length cDNA clones of Flk-1 were isolated from another day 8.5mouse embryo cDNA library, which had been prepared according to themethod of Okayama and Berg (1983), and a day 11.5 mouse embryo λgt11library (Clonetech) using the ³²P-labeled (Feinberg, A. P. andVogelstein, B. 1983 Anal. Biochem. 132:6-13) 210-bp PCR fragment.

6.1.2. Mouse Embryos

Balb/c mice were mated overnight and the morning of vaginal plugdetection was defined as ½ day of gestation. For Northern blot analysisthe frozen embryos were homogenized in 5 M guanidinium thiocyanate andRNA was isolated as described (Ullrich, A. et al., 1985, Nature313:756-761). For in situ hybridization, the embryos were embedded inTissue-Tek (Miles), frozen on the surface of liquid nitrogen and storedat −70C prior to use.

6.1.3. Preparation of Probes

The 5′-located 2619 bp of the receptor cDNA were subcloned in the pGem3Zvector (Promega) as an EcoR1/BamH1 fragment. The probe for Northern blothybridization was prepared by labelling the cDNA fragment with α-³²PDATP(Amersham) by random hexanucleotide priming (Boehringer; Feinberg, A. P.and Vogelstein, B., 1983 Anal. Biochem. 132:6-13).

For in situ hybridization a single-strand antisense DNA probe wasprepared as described by Schnurch and Risau (Development, 1991111:1143-54). The plasmid was linearized at the 3′ end of the cDNA and asense transcript was synthesized using SP6 RNA polymerase (Boehringer).The DNA was degraded using DNAase (RNAase free preparation, BoehringerMannheim). With the transcript, a random-primed cDNA synthesis with aα-³⁵S dATP (Amersham) was performed by reverse transcription with MMLVreverse transcriptase (BRL). To obtain small cDNA fragments of about 100bp in average suitable for in situ hybridization, a high excess ofprimer was used. Subsequently the RNA transcript was partiallyhydrolyzed in 100 mM NaOH for 20 minutes at 70° C., and the probe wasneutralized with the same amount of HCl and purified with a Sephadex C50column. After ethanol precipitation the probe was dissolved at a finalspecific activity of 5×10⁵ cpm. For control hybridization a sense probewas prepared with the same method.

6.1.4. RNA Extraction and Northern Analysis

Total cytoplasmic RNA was isolated according to the acidic phenol-methodof Chromczynski and Sacchi (1987). Poly(A⁺) RNA aliquots wereelectrophoresed in 1.2% agarose formaldehyde (Sambrook, J. et al., 1989Molecular Cloning: A Laboratory Manual 2nd ed. Cold Spring HarborLaboratory Press) gels and transferred to nitrocellulose membranes(Schleicher & Schuell), Hybridizations were performed overnight in 50%formamide, 5×SSC (750 mM sodium chloride, 75 mM sodium citrate), 5×Denhardt's (0.1% Ficoll 400, 0.1% polyvinylpyrrolidone, 0.1% BSA) and−0.5% SDS at 42° C. with 1-3×10⁶ cpm-ml-¹ of ³²P-Random primed DNAprobe, followed by high stringency washes in 0.2×SSC, 0.5% SDS at 52° C.The filters were exposed for 4 to 8 days.

6.1.5. In Situ Hybridization

Subcloning postfixation and hybridization was essentially performedaccording to Hogan et al. (1986). 10 μm thick sections were cut at −18°C. on a Leitz cryostat. For prehybridization treatment no incubationwith 0.2M HCl for removing the basic proteins was performed. Sectionswere incubated with the ³⁵S-cDNA probe (5×10⁴ cpm/μ1) at 52° C. in abuffer containing 50% formamide, 300 mM NaCl, 10 mM Tris-HCl, 10 mMNaPO₄ (pH 6.8), 5 mM EDTA, 0.02% Ficoll 400, 0.01% polyvinylpyrrolidone,0.02% BSA, 10 m/ml yeast RNA, 10% dextran sulfate, and 10 mM NaCl, 10 mMTris-HCl, 10 mM NaPO₄ (pH 6.8), 5 mM EDTA, 10 Mm DTT at 52° C.). Forautoradiography, slides were coated with Kodak NTB2 film emulsion andexposed for eight days. After developing, the sections werecounterstained and toluidine blue or May-Grinwald.

6.1.6. Preparation of Antisera

The 3′ primed EcoRV/HindIII fragment comprising the 128 C-terminal aminoacids of Flk-1 was subcloned in the fusion protein expression vectorpGEX3X (Smith, D. B. and Johnson, K. S., 1990 Gene. 67:31-40;Pharmacia). The fusion protein was purified as described and used forimmunizing rabbits. After the second boost the rabbits were bled and theantiserum was used for immunoprecipitation.

6.1.7. Transient Expression of Flk-1 in COS-1 Cells

Transfection of COS-1 cells was performed essentially as described byChen and Okayama (1987 Mol. Cell. Biol. 7:2745-2752) and Gorman et al.(1989 Virology 171:377-385). Briefly, cells were seeded to a density of1.0×10⁶ per 10-cm dish and incubated overnight in DMEM containing 10%fetal calf serum (Gibco). 20 μg of receptor cDNA cloned into acytomegalovirus promotor driven expression vector was mixed in 0.5 ml of0.25 M CaCa₂, 0.5 ml of 2×BBS (280 mM NaCl, 1.5 mM Na₂HPO₄, 50 mM BES,pH 6.96 and incubated for 30 min at room temperature. The calciumphosphate/DNA solution was then added to the cells, swirled gently, andincubated for 18 hours at 37° C. under 3% CO₂. For ligand bindingexperiments, the cells were removed from the plate and treated asdescribed below.

To obtain VEGF conditioned media, cells were transfected in 15-cmdishes. Media was collected after 48 h and VEGF was partially purifiedby affinity chromatography using heparin High Trap TM columns(Pharmacia) and concentrated by ultrafiltration (Ferrara, N. and Henzel,W. J. 1989 Biochem. Biophys. Res. Comm. 161:851-858). The concentrationof VEGF was determined by a ligand competition assay with bovine aorticendothelial cells.

For autophosphorylation assays, cells were seeded in 6-well dishes(2×10⁵ cells per well), transfected as described above, and starved for24 h in DMEM containing 0.5% fetal calf serum. The cells were thentreated with 500 pM VEGF for 10 min. at 37° C. or left untreated andwere subsequently lysed as described by Kris et al. (1985). Flk-1 wasimmunoprecipitated with an antiserum raised in rabbits against theC-terminus of the receptor. The immunoprecipitates were separated on a7.5% SDS polyacrylamide gel, transferred to nitrocellulose, andincubated with a mouse monoclonal antibody directed againstphosphotyrosine (5E2; Fendly, B. M. et al., 1990 Cancer Research50:1550-1558). Protein bands were visualized using horseradishperoxidase coupled goat anti-mouse antibody and the ECL™ (Amersham)detection system.

6.1.8. Radioiodination of VEGF

Recombinant human VEGF (5 μg; generously provided by Dr. H. Weich) wasdissolved in 110 μl sodium phosphate buffer pH 76, and iodinated by theprocedure of Hunter and Greenwood (1962). The reaction products wereseparated from the labeled protein by passage over a sephadex G50column, pre-equilibrated with phosphate buffered saline (PBS) containing0.7% bovine serum albumin (BSA), and aliquots of the collected fractionswere counted before and after precipitation with 20% trichloraceticacid. The purity of the iodinated product was estimated to be superiorto 90%, as determined by gel electrophoresis, and the specific activitywas 77000 cpm/ng. The bioactivity of the iodinated VEGF was confirmed bycomparison with the bioactivities of native VEGF using the tissue factorintroduction assay described by Clauss, M. et al. (1990 J. Exp. Med.172:1535-1545).

6.1.9. Crosslinking of VEGF to Flk-1

COS-1 cells transiently expressing Flk-1 and untransfected COS-1 cellswere incubated with 200 pM ¹²⁵I-VEGF at 4° C. overnight, then washedtwice with PBS and exposed to 0.5 mM disuccinimidyl suberate (DSS) inPBS for 1 h at 4° C. The cells were lysed, Flk-1 immunoprecipitated, andanalyzed by electrophoresis on a 7% polyacrylamide gel followed byautoradiography.

6.1.10. VEGF Binding

Ligand binding experiments were performed as described previously(Schumacher, R. et al., 1991, J. Biol. Chem. 266:19288-19295), COS-1cells were grown in a 15-cm culture dish in DMEM for 48 h aftertransfection. Cells were then washed carefully with PBS and incubatedwith 5 ml of 25 mM EDTA in PBS for 10 min. Cells were then removed fromthe plate, washed once with binding buffer (DMEM, 25 mM HEPES, pH 7.5,0.15% gelatin) and resuspended in 5 ml of binding buffer to determinethe cell number. In a total volume of 500 μl this cell suspension wasincubated for 90 min at 15° C. with 10 pM ¹²⁵I-VEGF, and increasingconcentration of unlabeled ligand (from 0 to 7×10⁻⁹), which waspartially purified from conditioned media of COS-1 cells transientlyexpressing VEGF (164 amino acid form; Breier et al., Development vol.114 (2) pp. 521-532 (1992). Leung et al., (Science vol. 246 pp. 1306-9(1989) disclose cDNA clones for bovine and human VEGF). Afterincubation, cells were washed with PBS 0.1% PBS in the cold. Free ligandwas removed by repeated centrifugation and resuspension in bindingbuffer. Finally, the ¹²⁵I radioactivity bound to the cells weredetermined in a gamma counter (Riastar). Data obtained were analyzed bythe method of Munson, P. J. and Rodbard, D. (1980 Anal. Biochem.107:220-235).

6.1.11. Retroviral Vectors Encoding Transdominant-Negative Mutants ofFlk-1

Recombinant retroviral vectors were constructed that contained thecoding region for amino acids 1 through 806 of the Flk-1 receptor (pLXFlk-1 TM cl.1 and pLX Flk-1 TM cl.3, FIG. 12A). A recombinant viruscontaining the 541 N-terminal amino acids of the CSF-1 receptor/c-fms(pNTK cfms TM cl.7, FIG. 12B) was used as a control.

pLXSN Flk-1 TM was obtained by ligating the 5′-located 2619 bp of theFlk-1 cDNA encoding amino acids 1 to 806 as a ClaI/BamHI fragment to aBglII/HpaI linker, thereby designing a stop-codon 23 amino acidsfollowing the transmembrane region (5′ GTC ATG GAT CTT CGT TAA 3′). In asecond step, the ClaI/HpaI fragment was subcloned into the ClaI/HpaIsite of the pLXSN vector. Stable GP+E-86 cell lines producing ecotropicretroviruses expressing the wild type and mutated receptor constructswere generated as described by Redemann et al. (Mol. Cell Biol. vol 12,p. 491-498 (1992)).

For generation of pNTK c-fms TM, a stop codon was introduced behindamino acids 541 downstream from the transmembrane region of the c-fmscDNA using the oligonucleotide 5′ TTG TAC AAG TAT AAG TAG TAG CCC AGGTAC CAG 3′. The mutated receptor was subcloned in the retroviralexpression vector pNTK2 (Stewart et al., EMBO J., 6, 383-388 (1987).Stable GP+E-86 cell lines were obtained as described above.

6.1.12. The Capability of FLK-1 TM to Form Signalling-IncompetentHeterodimers

The capability of Flk-1 TM to form signalling incompetent heterodimerswith the 180 kD wild type Flk-1 was demonstrated by coprecipitation ofthe truncated 130 kD receptor mutant with an antibody against theC-terminus of the intact receptor from lysates of COS cells transientlyexpressing both forms. Since the antibody could not recognize Flk-1 TM,coprecipitation was a consequence of heterdimerization.

To test the capability of Flk-1 TM to form signalling incompetentheterodimers with the wild-type Flk-1 in vivo, C6 gliobastoma tumorcells, available from the ATCC, accession number CCL 107, were implantedinto nude mice either alone or coimplanted with virus producing cells.Injected cell numbers for the two sets of experiments are indicatedbelow. Beginning at the time when the first tumors appeared, tumorvolumes were measured every 2 to 3 days to obtain a growth curve. Theresults are discussed in Section 6.2.6 and shown in FIGS. 12 and 13.

Experiment No. 1

Number of Number of Virus-Producer Number of Mice C6 Cells Cell LineVirus-Cells 4 5 × 10⁵ pLXSN Flk-1 TM cl.3 1 × 10⁷ 4 5 × 10⁵ None 0 4 5 ×10⁵ pNTK cfms TM cl.7 5 × 10⁶Experiment No. 2

Number of Number of Virus-Producer Number of Mice C6 Cells Cell LineVirus-Cells 4 2 × 10⁶ pLXSN Flk-1 TM cl.1 2 × 10⁷ 4 2 × 10⁶ pLXSN Flk-1TM cl.3 2 × 10⁷ 4 2 × 10⁶ None 0 4 2 × 10⁶ pNTK cfms TM cl.7 2 × 10⁷

In another experiment, the same experimental conditions were performedexcept that the virus producing cells were injected five days afterimplantation of 10⁶ tumor cells.

In another experiment, co-implantation of C6 glioblastoma cells was withdifferent relative amounts of retroviral cells producing comparabletiters (1×10⁶ cfu/ml) of recombinant retrovirus. The effect of theinhibition of tumor growth was dose-dependent, with maximum achievedwhen the virus-producing cells were in 20-fold excess over the tumorcells. To confirm that the inhibition of the C6 glioblastoma growth wascaused by dominant-negative action of the retrovirally expressedconstructs on endothelial cells, the tumors were resected and analyzed.Comparison of the whole mount specimens revealed striking differences:whereas the control tumors exhibited a reddish surface, as expected forwell-vascularized tissue, the inhibited cell implants were very pale.Histological staining of frozed sections revealed that the controltumors consisted of a homogenous mass of viable cells. Only very few andsmall necroses could be detected. In contrast, the much smaller,growth-inhibited tumor cell implants had an onion-like histologicalappearance, which was characterized by different tissue layers: a large,central necrosis was surrounded by a dense layer of viable tumor cells.Invasion of this tumor had not progressed, as evidenced by the presenceof natural structures of the skin, such as the muscular cell layer.

The distribution of capillaries and blood vessels in the tissuespecimens was determined by incubating frozen tissue sections with a ratmonoclonal antibody specific for the endothelial cell adhesion antigenPECAM (De Vries et al., Science vol. 255, pp. 989-991 (1992)). While thetumors coimplanted with the control virus-producing cells displayed thepattern of capillaries and vessels expected for well-vascularizedtissue, the growth-inhibited tumor cell implant exhibited a largecentral tumor cell necrosis, which was surrounded by a layer of viabletumor cells lacking blood vessels or capillaries. The tumor cells inthis layer showed a higher cell density than the control tumorsuggesting a significant reduction in tumor-induced edema formation.Since VEGF appears to induce vascular permeability in vivo, and wastherefore also designated vascular-permeability factor, inhibition ofVEGF/Flk-1 interaction may inhibit tumor associated edema formation.

Flk-1 expression in proliferating endothelial cells of the tumor wasconfirmed by in situ hybridization of tissue sections with a ³⁵S-labeledFlk-1 specific antisense cDNA probe and displayed the same distributionas immunostaining with endothelial cell-specific antibodies, indicatingthat proliferating endothelial cells expressed Flk-1. In situhybridization with a neomycin resistance gene (neo^(r)) antisense probeconfirmed the presence of retroviral sequences. The entire Flk-1dominant-negative-inhibited tumor consisting of the retrovirus-producingand infected cells was neo^(r)-positive, a result that exactly matchedthat obtained with a Flk-1 specific probe. The morphology of tumors thathad been coimplanted with control virus-producing cell was very similar,but the virus-producing cells were extensively infiltrated by infectedtumor cells. In these tumors, which contained many capillaries and bloodvessels, neo^(r)-positive signals were also found in endothelial cells.

C6 gliomas exhibit morphological characteristics of human glioblastomamultiforme such as necroses with palisading cells, a high degree ofvascularization, and a similar expression pattern of VEGF and itsreceptors making this model an excellent tool to study anti-angiogenictherapy (Plate et al., Cancer Research vol. 53, pp. 5822-5827 (1993)).

6.1.13. Intracerebral Grafting of Glioma Cells

To test the capability of Flk-1 TM to form signalling incompetentheterodimers with the wild type Flk-1 in vivo, C6 glioma cells weretransplanted intracranially into syngeneic rats with co-injection of aretrovirus-producing cell line.

To transplant glioma cells intracranially into rats, the rats(bodyweight 160-180 g) were anesthetized by i.p. injection of 100 mg/kgKetamin (Ketaset^(R)) plus 5 mg/kg (Xylazine (Rompun^(R)). The dosage isdependent on the rat strain and should be determined before theexperiments. Preanesthesia with isoflurane facilitates i.p. injectionand onset of anesthesia. Approximately 3-10 minutes after i.p.injection, animals no longer respond to pain. If analgesia isnot-complete after 10 minutes, additional dose of 50 mg/kg Ketanest plus2.5 mg/kg Rompun i.m. (50% of the initial i.p. dose) should be injected.The animals were adjusted in a commercially available small animalstereotactic apparatus. The skin was cleaned with alcohol and a medianincision (approximately 1 cm in length) was performed over the skullusing a sterile surgical blade. The skin was then slightly disattachedfrom the skull. A burrhole was made using a dental driller on the rightside of the hemisphere (coordinates: 2 mm lateral and 1 mm anterior tothe bregma). Care was taken not to disrupt the meninges or to damage thebrain (check via microscope). A hamilton syringe was placed in theburrhole at the level of the arachnoidea and then slowly lowered untilthe tip is 3 mm deep in the brain structure (the target point is thecauda-putamen, lateral to the frontal horn of the lateral ventricle).Two to twenty microliters of cells (depending of the amount of cells onewishes to graft) were slowly injected. The maximum volume rats tolerateis 20-25 microliters. If a higher volume is injected, the animal can dieimmediately due to increased intracranial pressure. After application ofthe cells, the syringe was not removed immediately in order to allowdissolution of the cells in the brain. After approximately 30 seconds,the syringe was removed slowly. Under these conditions no or very littleliquid comes out of the burrhole. The skull was then cleaned and theanimals did not bleed at the area operated (check via microscope). Theskin was then closed with sutures. Approximately 20-30 minutes wasneeded for intracerebral grafting of tumor cells in one animal (plusanesthesia).

Animals are monitored for 18-22 days at which time surviving rats aresacrificed, their brains removed (quick frozen or fixed in formalin) andanalyzed by standard techniques for measuring tumor volume.

Experiment No. 1

Number of Number of Virus-Producer Number of Rats C6 Cells Cell LineVirus-Cells 8 5 × 10⁴ Flk-1 TM 5 × 10⁶ 8 5 × 10⁶ 0 0

All cells were placed intracerebrally in a total volume of ≦25 μl.

The results of this experiment are described in Section 6.2.6 and shownin FIGS. 16A and 16B.

6.1.14. Assay for and Identification of Organic Compounds that ModulateFLK-1 Mediated Signal Transduction

Organic compounds that modulate Flk-1 receptor mediated signaltransduction can be assayed in a cellular Flk-1 assay wherein modulationof Flk-1 receptor autophosphorylation is measured using anantiphosphotyrosine antibody. In the example shown below, the resultsare analyzed using a Western blot of electrophoresed cell lysates.Levels of phosphorylation as be measured by other techniques known inthe art.

NIH3T3 cells expressing high levels of Flk-1 were seeded in 12-wellplates at 250,000 cells/well in DMEM+0.5% calf serum and incubatedovernight at 37° C. plus or minus the test substance. Flk-1 tyrosinekinase was stimulated by the addition of 100-500 pM of VEGF/well for5-10 minutes at 37° C. After stimulation, cells were washed withphosphate buffered saline (PBS) and then lysed with 200 μl of samplebuffer (100 mM Tris pH 6.8, 5% glycerol, 1.75% SDS, 1.25 mM EDTA, 0.5 mMsodium vanadate, 2.5 mM sodium pyrophosphate, 1.25% 2-mercaptoethanol).Cell lysates were transferred to centrifuge tubes, boiled at 100° C. for5 minutes, and then centrifuged at 16,000 G for 5 minutes. Supernatantswere removed and stored at −80° C.

For the Western blot assay, forty microliters of saved supernatant perlane were loaded onto a 7.5% SDS-PAGE gel (10 lanes/gel, 1.5 mm thick)and run at 120 V until the dye reached the bottom of the gel. Therunning buffer used contains 20 mM Tris, 192 mM glycine and 0.1% SDS.Proteins were then transferred to nitrocellulose membrane (Bio-Rad) at500 mA for 1 hour using ice-cold transblotting buffer containing 50%tank buffer, 20% methanol and 30% water. The nitrocellulose was thenblocked with 5% nonfat milk in TBST (50 mM Tris, 150 mM NaCl, and 0.1%triton) for 1 hour or overnight, immunoblotted with a monoclonalantibody against phospho-tyrosine (UBI or Sigma, 1:3000) in TBST for 1hour, followed by goat anti-mouse (Bio-Rad 1:3000) in TBST-buffer foranother 1 hour. Protein bands were detected by soaking the membrane inECL chemiluminescence system (Amersham Corp., prepared by mixing equalvolumes of reagent 1 and 2) for 1 minute and then exposing the film forabout 1-10 minutes.

6.1.15. Synthesis of a 3-PHENYL-1,4-DIAZA-ANTHRACENE

A preferred method of synthesis of AG1385 is as follows: 0.47 grams (3mM) 2,3-diaminonaphthalene and 0.47 grams phenyl gloxal hydrate in 20 mlethanol were refluxed 1.5 hour. Cooling and filtering gave 0.5 g (65%)of a light brown solid, mp 163° C. NMR CDCl3: δ 9.38 (1H, l.c., H2),8.71, 8.67 (2H, 2d, H5, 10), 8.25, 8.10 (4H, AA′BB′m, H6-9), 7.58(5H, m,Ph). MS: +256(M+, 100%), 229 (M-CN, 12%), 126(71%) m/e.

6.2. Results

6.2.1. Isolation of Flk-1

To identify RTKs that are expressed during mouse development, PCR assaysusing two degenerate oligonucleotide primer pools that were designed onthe basis of highly conserved sequences within the kinase domain of RTKswere performed (Hanks, S. K. et al. 1988, Science 241: 42-52). DNAextracted from a λgt10 cDNA library of day 8.5 mouse embryos (Fahrner,K. et al., 1987, EMBO. J., 6: 1497-1508), a stage in mouse developmentat which many differentiation processes begin was used as the templatein the PCR assays. In a parallel approach, with the intention ofidentifying RTKs that regulate angiogenesis, similar primers were usedfor the amplification of RTK cDNA sequences from capillary endothelialcells that had been isolated from the brains of postnatal day 4-8 mice,a time at which brain endothelial cell proliferation is maximal(Robertson, P. L. et al., 1985, Devel. Brain Res. 23: 219-223). Bothapproaches yielded cDNA sequences (FIG. 11, SEQ. ID NO. 7) encoding therecently described fetal liver RTK, Flk-1 (Matthews, W. et al., 1991,Proc. Natl. Acad. Sci. U.S.A. 88: 9026-9030). Based on amino acidhomology, this receptor is a member of the type III subclass of RTKs(Ullrich, A. and Schlessinger, J. 1990, Cell 61: 203-212) and is closelyrelated to human fit, which also contains seven immunoglobin-likerepeats in its extracellular domain in contrast to other RTKs of thatsubfamily, which contain only five such repeat structures (Matthews, W.et al., 1991, Proc. Natl. Acad. Sci. U.S.A. 88: 9026-9030). Sequencecomparisons of Flk-1 with KDR (Terman, B. I. et al., 1991, Oncogene 6:1677-1683) and TKr-C (Sarzani, R. et al., 1992, Biochem. Biophys. Res.Comm. 186: 706-714) suggest that these are the human and rat homologuesof Flk-1, respectively (FIG. 1).

6.2.2 Expression of Flk-1 mRNA During Embryonic Development

As a first step towards the elucidation of the biological function ofFlk-1, the expression of Flk-1 mRNA was analyzed in mouse embryos atdifferent development stages. Northern blot hybridization experimentsindicated abundant expression of a major 5.5 kb mRNA between day 9.5 andday 18.5, with an apparent decline towards the end of gestation (FIG.2A). In postnatal day 4-8 brain capillaries Flk-1 mRNA was found to behighly enriched compared to total brain mRNA (FIG. 2B).

In situ hybridization experiments were performed to obtain more detailedinformation about the expression of Flk-1 during different embryonalstages. A single-stranded antisense, 2619-nucleotide-long DNA probecomprising the Flk-1 extracellular domain was used as a probe because itgenerated the most specific hybridization signals. As an example, aparasagittal section of a day 14.5 embryo is shown in FIGS. 3A, 3 b and3C. High levels of hybridization were detected in the ventricle of theheart, the lung, and the meninges; other tissues such as brain, liver,and mandible appeared to contain fewer cells expressing Flk-1 mRNA. Thinstrands of Flk-1 expression were also observed in the intersegmentalregions of the vertebrae and at the inner surface of the atrium and theaorta. Higher magnification revealed that the expression of Flk-1 seemedto be restricted to capillaries and blood vessels. Closer examination ofthe heart, for example, showed positive signals only in the ventricularcapillaries and endothelial lining of the atrium (FIG. 4A). In the lung,Flk-1 expression was detected in peribronchial capillaries, but wasabsent from bronchial epithelium (FIG. 4D). The aorta showed stronghybridization in endothelial cells, but not in the muscular layer (FIG.4C).

6.2.3. Expression of Flk-1 During Organ Angiogenesis

The neuroectoderm in the telencephalon of a day 11.5 mouse embryo islargely avascular; the first vascular sprouts begin to radially invadethe organ originating from the perineural vascular plexus (Bar, J.,1980, Adv. Anat. Embryol. Cell. Biol. 59:1-62; Risau, W. and Lemmon, V.1988, Dev. Biol. 125:441-450). At this stage, expression of Flk-1 washigh in the perineural vascular plexus and in invading vascular sprouts,as shown in FIG. 5A. These in situ hybridization analyses indicated thatthe proliferating endothelial cells of an angiogenic sprout expressedthe Flk-1 mRNA. At day 14.5, when the neuroectoderm is already highlyvascularized, numerous radial vessels as well as branching vessels ofthe intraneural plexus contained large amounts of Flk-1 mRNA (FIG. 5B).At postnatal day 4, when sprouting and endothelial cell proliferation isat its highest, strong expression of Flk-1 mRNA was observed inendothelial cells (FIG. 5C). Conversely, in the adult brain whenangiogenesis has ceased, Flk-1 expression was very low (FIG. 5D) andappeared to be restricted mainly to the choroid plexus (FIGS. 6A and6B). In the choroid plexus, cells in the inner vascular layer expressedFlk-1 mRNA, while epithelial cells did not (FIG. 6A, 6B).

The embryonic kidney is vascularized by an angiogenic process (Ekblom,P. et al., 1982, Cell Diff. 11:35-39). Glomerular and peritubularcapillaries develop synchronously with epithelial morphogenesis. In thepostnatal day 4 kidney, in addition to other capillaries, prominentexpression of Flk-1 was observed in the presumptive glomerularcapillaries (FIG. 7A). This expression persisted in the adult kidney(FIGS. 7C and 7D) and then seemed to be more confined to the glomerularcompared to the early postnatal kidney.

6.2.4. Flk-1 Expression in Endothelial Cell Progenitors

To investigate the possible involvement of Flk-1 in the early stages ofvascular development, analysis of embryos at different stages duringblood island formation were performed. In a sagittal section of thedeciduum of a day 8.5 mouse embryo, Flk-1 expression was detected onmaternal blood vessels in the deciduum, in the yolk sac and in thetrophectoderm. Flk-1 mRNA was also found in the allantois and inside theembryo, mainly located in that part where mesenchyma is found (FIG. 8A).At a higher magnification of the maternal deciduum, high levels of Flk-1mRNA expression were found in the inner lining of blood vessels, whichconsist of endothelial cells (FIG. 8B). In the yolk sac, hybridizationsignals were confined to the mesodermal layer, in which thehemangioblasts differentiate (FIG. 8C). FIG. 8D shows a blood island athigher magnification, in which the peripheral angioblasts expressed ahigh level of Flk-1 mRNA.

6.2.5. Flk-1 is a High Affinity Receptor for VEGF

Detailed examination of in situ hybridization results and comparisonwith those for VEGF recently reported by Breier, G. et al. (1992,Development 114:521-532) revealed a remarkable similarity in expressionpattern. Furthermore, Flk-1 expression in the glomerular endothelium andVEGF in the surrounding epithelial cells (Breier, G. et al., 1992,Development 114:521-532) raised the possibility of a paracrinerelationship between these cells types and suggested therefore aligand-receptor relationship for VEGF and Flk-1, respectively. In orderto test this hypothesis, the full-length Flk-1 cDNA was cloned into themammalian expression vector PCMV, which contains transcriptional controlelements of the human cytomegalovirus (Gorman, C. M. et al., 1989,Virology 171:377-385). For transient expression of the receptor, theFlk-1 expressing plasmid was then transfected into COS-1 fibroblasts.

Specific binding of VEGF to the Flk-1 RTK was demonstrated bycrosslinking and competition binding experiments. Purified¹²⁵I-labeled-VEGF was incubated with COS-1 cells transfected with thepCMV-Flk-1 expression vector. Crosslinking with DSS and subsequentanalysis of immunoprecipitation, PAGE, and autoradiography revealed anapproximately 220 kD band which was not detected in the controlexperiment with untransfected COS-1 cells and is likely to represent theVEGF/Flk-1 receptor complex (FIG. 9A). In addition, VEGF competed with¹²⁵I-VEGF binding to Flk-1 expressing COS-1 cells (FIG. 9B), whereasuntransfected COS-1 cells did not bind ¹²⁵I-VEGF. The interaction ofVEGF with the receptor on transfected cells was specific, as PDGF-BB didnot compete with binding of ¹²⁵I-VEGF. Analysis of the binding datarevealed a Kd of about 10⁻¹⁰ M, suggesting that Flk-1 is a high affinityreceptor of VEGF. This finding, together with the Flk-1 and VEGF in situhybridization results strongly suggests that Flk-1 is a physiologicallyrelevantly receptor for VEGF.

An autophosphorylation assay was performed to confirm the biologicalrelevance of VEGF binding to the Flk-1 receptor. COS1 cells whichtransiently expressed Flk-1 were starved in DMEM containing 0.5% fetalcalf serum for 24 h, stimulated with 0.5 mM VEGF, and lysed. Thereceptors were immunoprecipitated with the Flk-1 specific polyclonalantibody CT128, and then analyzed by SDS-PAGE and subsequentimmunoblotting using the antiphosphotyrosine antibody 5E2 (Fendly, B. M.et al., 1990, Cancer Research 50:1550-1558). A shown in FIG. 10, VEGFstimulation of Flk-1 expressing cells led to a significant induction oftyrosine phosphorylation of the 180 kD Flk-1 receptor.

6.2.6. Inhibition of Tumor Growth by Transdominant-Negative Inhibitionof Flk-1

The Flk-1 receptor is believed to play a major role in vasculogenesisand angiogenesis. Therefore, inhibition of Flk-1 activity may inhibitvasculogenesis of a developing tumor, for example, and inhibit itsgrowth.

The dominant-negative potential of Flk-1 TM was first examined bymeasuring its influence on the mitogenic response of Flk-1-expressingNIH 3T3 cells after superinfection with the Flk-1 TM virus. [³H]thymidine incorporation in the 3T3 Flk-1 cell line was maximallystimulated by 500 pM VEGF, with an EC₅₀ of about 100 pM. Aftersuperinfection with the Flk-1 TM virus, the Flk-1/VEGF-mediatedmitogenic response was dramatically suppressed even at a ligandconcentration of 5 nM. While 3T3 Flk-1/Flk-1 TM cells expressed wildtype Flk-1 levels equal to the parental line, they displayed, due tooverexpression of Flk-1 TM, a 6-fold increase of cell surface receptors,as determined by ¹²⁵I-VEGF binding. These results were further extendedby Flk-1 TM virus-induced suppression of Flk-1 transforming activity anddemonstrated not only that mutant and wild type Flk-1 physicallyassociated, but also that this interaction generatedsignalling-incompetent heterodimers. The dominant-negative inhibitoryeffect which was achieved at a fivefold excess of Flk-1 TM could not beovercome by a 50 fold ligand excess relative to the EC₅₀ value formitogenic activation. Moreover, Flk-1 TM did not interfere with thesignal transduction of the α- and β-PDGF-receptors, demonstrating thespecificity of its dominant-negative action.

To test the dominant-negative potential in vivo, tumor cells (C6 ratglioblastoma) and mouse cells producing a recombinant retrovirusencoding a truncated Flk-1 receptor were mixed and implantedsubcutaneously into nude mice. The implanted C6 glioblastoma cellssecrete VEGF which will bind to and activate the Flk-1 receptorsexpressed on the surface of mouse endothelial cells. In the absence ofany inhibitors of vasculogenesis, the endothelial cells will proliferateand migrate towards the tumor cells. Alternatively, if at the time ofinjection, the tumor cells are co-injected with cells producingrecombinant retrovirus encoding the dominant-negative Flk-1, or if thecells producing recombinant retrovirus are injected after the tumorcells, the endothelial cells growing towards the implanted tumor cellswill become infected with recombinant retrovirus which may result indominant-negative Flk-1 mutant expression and inhibition of endogenousFlk-1 signaling. Suppression of endothelial cell proliferation andmigration will result in failure of the implanted tumor cells to becomevascularized which will lead to inhibition of tumor growth. As shown inFIGS. 12, 13 and 15 tumor growth is significantly inhibited in micereceiving implantations of cells producing truncated Flk-1 indicatingthat expression of a truncated Flk-1 receptor can act in adominant-negative manner to inhibit the activity of endogenous wild-typeFlk-1.

As a control, any direct influence of the retroviruses on the growth ofthe tumor cells can be excluded by growing C6 cells in conditioned mediaof the different retrovirus-producing cell lines, without any effect ontheir growth behavior.

To test the role that Flk-1 receptor is believed to play in angiogenesisand vasculogenesis, and to identify potential inhibitors of the Flk-1receptor, tumor cells (C6 rat glioblastoma) and mouse cells producing arecombinant retrovirus encoding a truncated Flk-1 receptor were mixedand implanted intracranially in rats. The implanted C6 glioblastomacells secrete VEGF which will bind to and activate the Flk-1 receptorsexpressed on the surface of rat endothelial cells. Inhibition of Flk-1receptor signal transduction is measured as inhibition of intracraniallytumor growth as seen in rats co-injected with the Flk-1 TM.

Analogous experiments with a variety of other tumor types support thedata obtained with the C6 glioblastoma nude mouse subcutaneous model andstrongly demonstrate that inhibition of solid tumor growth can beeffected by preventing angiogenesis. Angiogenesis a process that isnormally regulated by VEGF, which when secreted by tumor cells attractsand stimulated in a paracrine fashion Flk-1-positive vascularendothelial cells.

6.2.7. Identification of an Organic Compound that Inhibits FLK-1Receptor Phosphorylation

To identify organic compounds that inhibit Flk-1 receptor, organiccompounds have been tested in the cellular assay described in Section6.1.14 for their ability to inhibit Flk-1 receptor phosphorylation.Examples of some of the compounds tested are shown below.

Compound A14 was shown to inhibit Flk-1 receptor phosphorylation almostcompletely at a test concentration of 100 μM (FIG. 17). Thus this is anexample of a compound that could be useful for antagonizing Flk-1receptor signal transduction and therefore may be useful for inhibitingthe Flk-1 receptor mediated mitogenic signal. Accordingly, compound A14may be therapeutically useful in the treatment of solid tumors byinhibiting angiogenesis.

The present invention is not to be limited in scope by the exemplifiedembodiments which are intended as illustrations of single aspects of theinvention, and any clones, DNA or amino acid sequences which arefunctionally equivalent are within the scope of the invention. Indeed,various modifications of the invention in addition to those describedherein will become apparent to those skilled in the art from theforegoing description and accompanying drawings. Such modifications areintended to fall within the scope of the appended claims.

It is also to be understood that all base pair sizes given fornucleotides are approximate and are used for purposes of description.

All references cited herein are hereby incorporated by reference intheir entirety.

1. A cell line that comprises a recombinant vector and expressestruncated Flk-1, wherein said vector comprises a nucleotide sequencethat encodes a truncated Flk-1, wherein the encoded polypeptide rendersendogenous wild-type Flk-1 unresponsive to VEGF which inhibits thecellular effects of VEGF binding and, wherein said truncated Flk-1consists of an amino sequence corresponding to 1-806 of SEQ ID NO:
 2. 2.A cell line according to claim 1, wherein said vector is a retrovirusvector, and wherein said cell line (i) produces infectious retrovirusparticles encoding truncated Flk-1 and (ii) expresses truncated Flk-1encoded by said retrovirus vector.